The self discharge rate of your batteries comes into play here. Unfortunately, that rate changes over time as sulfates build up in the battery & I don't know a good simple way to measure it accurately without investing in some equipment. If your little solar maintainer does not put out enough amps to overcome the self discharge rate, then it is not going to help you. In a 12v system, each watt only puts out roughly 0.08 amps, when the solar panel sees full sun. You are not going to have even close to full sun in the winter, so you will get less than that. In my opinion, anything less than 15 watts isn't worth bothering with & even at that size, a battery with a bad self discharge rate will still run itself (& the rest of the bank) down. Around 50 watts is where things start to get more real. You might actually be able to get an amp out of a 50w system in the winter, assuming that the panel isn't covered with snow.
As was already said, having any solar panel wired up without a controller is a dangerous thing to do, especially long term & even more especially when not monitored.
How about taking the battery out of the boat & sticking it in a utility room somewhere with a maintainer that plugs into a wall socket? That's what I always did when I lived up north.