Depending on the weather, the water from launching may have found its way into the caliper and froze there. Water expands as it changes from liquid to ice. This is what bursts pipe. This expansion could have forced the pad against the rotor and firmly 'applied' the brakes.
Another possibility is that you have water in your brake lines. This water would effectively do the same thing. Freezing, expanding and raising the pressure in the brake like, firmly 'applying' the brakes.
You might have the fluid changed if you suspect it is contaminated. Pulling the trailer around the parking lot and circling left and right will help to heat the rotors up and help slow freezing until enough water has dripped off so as to not be a problem. Same thing after loading. You should apply the brakes manually a few times gently while pulling away after pulling the boat out. This will put a little heat on the rotors and keep that water from freezing right away.
Hope this helps.
Quit complaining about the color, just pull up your skirt and fish! -- snagged