The lifejacket is the decision that matters most
Owners often skip a lifejacket because their dog swims at the beach. Open water from a boat is different: there is a drop into the water, a moving hull he cannot climb, possible cold shock, swell or current, and a real chance he fell in because he slipped or was startled, not because he chose to swim. A well-fitted canine lifejacket keeps him afloat with his head up while he is frightened and tired, makes him visible with bright colours and reflective strips, and — most importantly — has a sturdy handle on the back so one person can haul him back aboard. A wet medium dog is heavy and wriggling; without that handle, recovery can fail.
Fit the jacket snugly so it cannot ride up over his head, with straps clear of his throat and legs moving freely. Practise at home and in shallow water first: wearing it, walking in it, and being lifted briefly by the handle, rewarded calmly, so none of it is new on the day he actually needs it.
Set up the boat so he cannot slip, stray or overheat
Walk the boat from your dog's perspective before you cast off. Netting or a lead arrangement should close the gaps he could squeeze through at the rails, stern and boarding gate, and he needs a defined resting place out of walkways, away from winches, ropes under tension and the helm's moving parts. Decks get hot enough in sun to burn paws, and they get slick when wet. A damp mat or towel in shade gives him grip and a cool surface; test any deck with the back of your hand before expecting him to stand on it, and move him off hot surfaces rather than relying on him to move himself.
Provide constant fresh water and real shade, because dogs overheat quickly on reflective water with no breeze at deck level. Plan toilet breaks on shore into your route rather than expecting him to hold on through a long passage, and give him a short, boring familiarisation trip before a full day out, so motion, engine noise and harbour sounds are not all new at once. If he is miserable, drooling, pacing or trying to hide, shorten the trip — seasickness and panic are safety issues on a boat, not inconveniences.
If your dog goes overboard
Shout clearly so the helm knows, point at the dog continuously and never take your eyes off him, because a small head in swell disappears astonishingly fast. The boat should turn and approach slowly from downwind with the engine in neutral near the dog, never reversing toward him. Lift him by the lifejacket handle, ideally with a second person steadying, and once he is aboard check him over, warm and dry him, and watch for coughing, laboured breathing or exhaustion after swallowing water. If breathing seems abnormal or he remains distressed after a dunking, contact a veterinarian promptly once ashore.
Never let a dog swim tethered to the boat, and supervise any swim from a bathing platform closely, with a clear way out of the water he has used before.
Back at the marina
Marinas are busy, slippery, unfamiliar places where a loose dog can fall between pontoon and hull or vanish among identical boats. Keep the lead on from boat to shore, and keep his identification current. Should he slip away among the pontoons, turn to KinsLe: the Lost Pet Alert System sits inside his profile without charge, appears as a map listing, and enables whoever spots him along the walkways to contact you by direct message.