Kallitype's new Adventure

Those are pretty handy.  It's a through-deck wire seal.  Ancor Marine makes them and I know West Marine carries them.  Here's the link.

I can't remember the size I used (if needed I can go measure it for you) but the hitch is you have to be able to get the "plug end" of your depthfinder cable through the fixture.  You end up having to use one bigger than you think you would really need and thus the generous dab of silicon on the top to make sure its water tight.  Very clean and easy way to run your wires. I used them on both the depthfinder and the chartplotter.  I ordered mine from Hookone but if West Marine has them that would save you shipping.

I also would recommend marine grade electrical connectors (West Marine should have them) and the generous use of liquid electrical tape.  Only way I have found to slow the evil hand of saltwater corrosion

http://kayakfishinggear.com/12liquidtitefitting.aspx

Congrats on the new boats, hopefully they took care of you. 
Instead of using a ton of silicone you can also use some self sealing 20 Mil  or similar tape to fill in the gap between the wires and the seal.  It can also be used as a waterproof version of electrical tape. 

"took care"  10% off the 2010 boat, and 10% off on accessories.  I'll pick up the boat at week's end.