I used:
1/2 tube of Plumber's Goop,
1 package of 6 water-resistant quick connects. (male and female) from the local auto parts store.
1 starfrit "lock-n-lock" brand container for the amp.
The battery I use came with my seascooter, you can just attach the power cables to the seadoo battery if you don't want the added expense of another battery.
4 nuts and bolts to attach the starfrit lid to the underside of the storage lid,
all in all, I think I paid about $25 for all the parts.
The longest part of the install is gooping the holes in the container and lids. I did it in stages,
I gooped the hole in the the storage bin lid with the wires coming through from the top first, waited 24 hours and then gooped the underside of the lid,
*** once the goop is cured, you cannot move the cables, so ensure you have enough length coming out the underside of the storage lid before you goop the top.
Then I attached the starfrit lid with the nuts and bolts, make sure you goop the bolts on the underside of the starfrit lid and also around the top of the storage cover where the bolts come through after you have it securely tightened. I used some styrofoam spacers between the starfrit lid and the storage bin lid, so as not to pinch the cables and wires.
I gooped all around the wires where they enter the top of the starfrit lid to be extra safe.
I was out this past Sunday and turned the ski over, by accident. It was under for about 20 seconds while I made sure the kids were ok. within 5 seconds of righting the ski, the music was back on. No water in the starfrit container where the amp is.

The glove box was full of water, luckily my MP3 player was also in a starfrit container.

Some call me anal, and it's after times like that where I'm glad that I am.
Use as much goop as you feel is necessary. My thought is better to put too much and not need it than not enough and need it after the fact. Nobody can see the goop on mine unless I remove the front storage lid. I hope this helps.
PS I also sprayed water across the gooped holes to ensure they were water-tight before moving on to the next step.