We had a 21 Stealth and it sounded very good for the money. When the boat was being built I talked with Martin, Ultra's stereo guy sorry I forget his shops name. Any way the boat came standard with 4 JL 6.5's, 2 JL 12's, and a JL six channel amp. I paid him direct to bump the subs to W3 12's (next step up from the W0's) and add a JL 500/1 to power the subs, two 4 ohm speakers wired in parrallel to 2 ohms, then I added to MB Quart 6 x9's in pods under the engine hatch off the 5th and sixth channel. I think we paid $2500 for the Big Ultra Stereo and then another $500 for the added amp and sub upgrade.
It sounded very clean and very loud for a 21 footer. The best feature was the added 6x9's under the hatch, they really put the sound out to the beach. The subs also hit very hard.
Anyway, that's what we ran. I have heard good things about the Polk/momo speakers. I'm not crazy about the Kenwood and Alpine amps. I was very impressed with the JL amps, never overheated on us once and they were buried under the side panel with little to no air movement.
I'm definately going to be running JL amps in the new boat.
Another argument you might hear is that if you want volume 6x9's are the way to go and if you want sound qual, then go with 6.5's. I think the key is to match good amps with good speakers and keep their placement high in the boat to get the sound out of the boat.
Anyway good luck and congrats on the new boat. You're gonna love owning an Ultra.
Oh yeah stick with the standard car audio stuff, no need to spend the money on Marine speakers.
Chris