1) Do not close all below the water seacocks until you determine if you cockpit drains empty below the waterline. Never close those.

2) If you are only using a 10 watt solar panel to charge two batteries a battery combiner is the wrong choice. The relay that joins the two batteries will drain close to the peak output of the solar panel. With short winter days your batteries may only be combined for a very short interval, if at all. You really need to determine if the 1-both-2 switch is wired the way they were originally intended or if the two batteries are permanently wired in parallel.

If they are permanently wired in parallel then all you have to do is connect the solar panel to the plus and minus terminals of either battery.

If the 1-both-2 switch is selecting the batteries independently then your decision is not as simple. Two batteries that are paralleled should have been paralleled since new. If not, they are not identical. The stronger battery will forever be discharging into the weaker one and in the end both will be dead. I would not recommend leaving the switch in the both position. So, if they are not permanently wired together the simplest option is to take one battery home and leave the solar charger on the other one.

One additional item. If you have an automatic bilge pump leave it on. Many boats get water in their bilge either from rain or small leaks such as prop shaft packing gland drips. No auto bilge pump? Check your bilges regularly!