For the most part, it's because ships are big things that tend to sink quickly once a lot of holes are punched into them. Tanks and planes have comparatively few people in them, and exit points are always nearby. On a ship, you can be working tens of metres from an exit to the deck, and there will usually be several watertight doors between you and your exit. If you don't have time to make it before the ship sinks, you're very unlikely to survive.

Additionally, the sea is a difficult place in which to survive. On land, it's easy to find your way to safety, while at sea, you need a compass. If you're in the middle of an ocean, rescue is unlikely to find you, while you'll have trouble living off the land until it does. If you've abandoned ship, and don't have a liferaft or lifeboat, then survival is even more unlikely. In some cases, crews and ships were equipped with insufficient lifesaving equipment - British ships in WW2 always needed more liferafts, and the lifebelts the crews were equipped with were almost more a hindrance than a help.