Have you verified that you do, in fact, get water into your bilge? You might be doing this all for naught.
However, I'm a big fan of the ProMariner brand. I've been familiar with them for 3 decades - being used on many boats as original equipment and they fare extremely well.
As noted above, close to the batteries is your answer for "where" to mount.
You can run it's power wire to something like a Marinco, hard-wired power inlet, which you can install through the deck in a convenient spot to plug-in your extension cord.