I just used a lifetime warranty starter from shucks and sealed the ends with a bit of silicone. Worked great for many years, then I went with a high torque starter for my current motor due to higher comp ratio and did the same with it.
The marine starters that I remember had either a rubber end gasket that didn't seal very well or nothing at all different than there automotive counterparts. Things may have changed as its been a few years since I worked at a marina.
jd