We saw some of the Drainman pumps on small boats in Europe last Fall.
It's an interesting idea, probably works well within reason, but would still require some attention and oversight.
Sometimes rains can occur without significant wave action, and real major storms can present conditions that would overwhelm the Drainman or an electric pump as well.
I'd sleep better 1.) under a covered berth with 2.) dual electric pumps and batteries, 3.) dock master oversight, and 4.) an occasional personal visit, especially after heavy storms. You can also add in canvas or fine net screens placed at the back entrance of the slip to keep blowing rain from coming under the enclosure, or simply add a solid, closing door to the arrangement.
The Boy Scouts were right on this one: "Be Prepared"!
Joe. :teeth :thup