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Late afternoon in the cove, three houseboats rafted up side by side. One runs its generator for the air conditioning, and the kids have pulled a floating mat right up to its stern because that’s where the ladder is. Nobody can see or smell anything wrong. That is exactly the picture safety agencies warn about. Carbon monoxide at the swim platform is one of the few boating dangers that can hurt people who never leave the water, and floating platforms and mats tend to end up right where it collects.
In short: engine and generator exhaust pools around the stern, the swim deck and the space under it. The CDC says CO there can kill someone in seconds, and the Coast Guard says to keep people off the back deck and swim platform while engines or a generator run. Keep your platform and mats away from any exhaust, at least 20 ft from boats running an engine or generator (the CDC’s distance), and fit a marine-rated CO alarm inside the boat.
Carbon monoxide and the swim platform danger zone
Carbon monoxide is produced by gasoline engines on boats, including onboard generators. It is colorless and odorless, and it gathers in predictable places. The CDC says CO that builds up in the air space beneath the stern deck, or on and near the swim deck, can kill someone in seconds, and that on larger boats it builds up above the water near the water platform. Houseboats and other large boats sometimes have generators that vent toward the rear. Slow speeds or idling can push CO into the cabin, cockpit, bridge and aft deck, and wind from behind the boat can make the build-up worse.
The Coast Guard adds two points people often miss: CO can accumulate anywhere in or around a boat when it is moving slowly, idling or stopped, and it can stay at dangerous levels even after your engine, or the other boat’s engine, has been shut off. Minnesota’s 2026 Boating Guide says swimmers have died when venturing too close to houseboats’ swim platforms while engines or generators were running.
Why platforms and mats belong away from exhaust
A floating platform puts people at water level, often right behind the boat, sometimes lying down. That is the zone both agencies describe. So the rules for platforms follow directly from what they publish:
- Engines and generators off while anyone is on the platform or in the water nearby. The Coast Guard says to keep people clear of the rear deck and swim platform while the generator or engines are running.
- Know where your exhaust outlets are (transom, side, or under the swim platform) and keep the float and swimmers away from them. The CDC says to swim and play away from areas where engines vent.
- Never go under the swim platform where exhaust outlets are unless the area has been properly ventilated (USCG).
- Mind your neighbors. Dock, beach or anchor at least 20 ft from the nearest boat running a generator or engine (CDC). In a raft-up, that includes the boat next to you.
- Watch children on swim decks and water platforms closely, as the CDC advises.
- No generator at anchor. Minnesota’s guide says not to leave a generator running at night or when anchored.
Tie the platform off the side or well back on a line, not snug against the transom, and never let it drift into the space under an overhanging swim deck. Our anchoring guide covers lines and setups.
Teak surfing and being dragged behind a boat
Teak surfing means hanging on to the swim platform while the boat moves and then body-surfing its wake. The Coast Guard says it is extremely dangerous because of carbon monoxide and sudden loss of consciousness, advises against it, and notes it is not illegal in some states. Minnesota’s guide says people have been poisoned while being towed, dragged or sitting too near the stern. The same applies to towing a platform or mat with people on it: RAVE says never to tow its dock with people or pets on board, and Aqua Lily Pad says its mats aren’t designed to be towed.
Symptoms: don’t call it seasickness
The CDC lists the most common symptoms of CO poisoning as headache, dizziness, weakness, nausea, vomiting, chest pain and confusion, and warns that CO can make you pass out, fall into the water and drown. The Coast Guard says to treat seasickness symptoms as possible CO poisoning: get the person into fresh air immediately and seek medical attention unless you are sure it isn’t CO. If you smell exhaust, CO is present too, although it can also be there with no smell at all.
| If you notice | Do this |
|---|---|
| Headache, dizziness, nausea or confusion in someone near the stern | Move them to fresh air, shut down engines and generator, get medical help |
| A swimmer who seems drowsy or confused near the platform | Get them out of the water and away from the stern; treat it as an emergency |
| Exhaust smell around the swim deck | Clear everyone from the area; find and fix the source before restarting |
| A CO alarm sounding | Never ignore it (USCG); open hatches, windows and ports to ventilate (Minnesota DNR) and move people to fresh air |
In an emergency, call 911 or use VHF channel 16; see first aid and emergencies.
How common is it?
In its 2024 recreational boating statistics, the Coast Guard recorded 5 incidents classed as carbon monoxide exposure, with 2 deaths and 26 injuries. Those are boating incidents only; swimmers who enter the water voluntarily from an anchored boat fall under the report’s “not reportable” categories, so no figure exists for platform use. The CDC and the Coast Guard both describe CO poisonings on boats as preventable.
CO alarms for the boat
The CDC recommends a working CO detector listed by Underwriters Laboratories as appropriate for marine use, installed inside the boat. The Coast Guard says to replace alarms as the manufacturer recommends, and Minnesota’s guide suggests marine-rated detectors supplemented by low-level detectors. An alarm protects the cabin; it can’t protect a swimmer at the transom, so the engine-off rule still comes first. Below are two marine CO alarms we found in stock: one runs on an internal battery, the other is wired to a 24 V boat system.
Fireboy-Xintex CO Alarm, Internal Battery
- Internal battery, no wiring (CMD6 line, retailer)
- CMD6 marine line: UL 2034 (retailer)
- About 7-year sensor life (retailer)
Safe-T-Alert 62 Series Marine CO Alarm, 24 V
- Conforms to UL 2034 (listing)
- Alarm point 400 ppm, 4-15 min
- End-of-life signal after 5 years
About $150 for the Fireboy-Xintex and about $155 for the Safe-T-Alert, seen October 2026 (the Safe-T-Alert had only a few left). The Safe-T-Alert shown is the 24 V version; most small boats run 12 V, so match your boat’s system and follow the maker’s mounting instructions.
Carbon monoxide questions
Can you get carbon monoxide poisoning while swimming?
Yes. The CDC says CO near the swim deck can kill in seconds and can cause people to pass out and drown. Swimmers near a running engine or generator exhaust are at risk.
How far should a floating platform be from a running boat?
The CDC says to dock, beach or anchor at least 20 ft from the nearest boat running a generator or engine. For your own boat, turn engines and generator off while people are on the platform.
Is it safe once the engine is off?
Safer, but the Coast Guard warns CO can remain at dangerous levels around a boat even after the engine stops. Give it time to clear, and stay away from other boats still running.
Do outboard boats have the same risk?
The CDC and Coast Guard warnings cover gasoline engines on boats in general, including onboard generators. Wherever the exhaust comes out, keep swimmers and floats away from it while it runs.
Is teak surfing illegal?
The Coast Guard says it is not illegal in some states, but it advises boaters never to do it because of CO poisoning and sudden loss of consciousness. Check your state’s rules.
Back to the water safety hub, or read swimming and supervision and Coast Guard requirements. Planning a stern setup? See boat swim platforms.