In a Boat or better yet a " Correctcraft
" you need a Total of 4 speakers a Head unit and if you want some Bass then a Sub. For Amps you can get away with a 4 channel for your mids/tweets and for the Sub amp you can either go a mono amp for high output Bass or get a 2 channel or 4 channel and bridge it ... if you are lost there get it then ask me whats bridging lol
depending on the size amps ask the retailer or person you buy your amps from for the correct size wire ( in boats you are likely to get radio buzz/interference ) since you have a lot of electronics like guages and etc on board.
Also I dont know if your supra has the cargo space behind the passenger seat or if you are going to be installing the amps and etc below the drivers dash panel.....My parents placed there head unit below the drivers dash panel along with the 2 amps . As for the Sub it went behind the passenger seat where most of the time ski's go. btw they had a 1999 Supra lol
Just look in the phone book for car audio dealers around you or if all else fails you can check crutchfield but they are $$$$$$$$$$$$$$ over priced imo. my other place craigslist.com
or ebay you can find some good amps there.
if you need any more help pm me
and good luck !