kotori

If you're worried about water getting in, put in an automatic bilge pump. It'll detect any dangerous amount of water and turn on, pumping out the water until the ship isn't in any danger. Standard practice in Model Warship Combat, where a little water from rudder tubes is the least of your concerns.

Umi gave good advice. Those're the easiest and cheapest things you can do, and it'll have the most effect. You only need a bilge pump if you're going into combat, seriously heavy seas, or you're on the water for days at a time.