Tom,
Yours is the first and only boat I've seen with compession-driver horn tweeters. It seems like these are the way to go, but the ID are a bit above my price range.
I have a 4 channel MTX amp I'm going to drive tower speakers with, and I'm leaning towards Skylon DeafCon III's. I'm still tinkering with the idea of building PVC cans.
These might make a pretty cool 6 1/2" horn "can":
Pyle PH65
Pyramid TWD80
It looks like the horn will fit in a standard 6 1/2" opening.
Specs:
Titanium Diaphragm
Gold Plated Banana Plugs
1.75" Copper Voice Coil
40 oz Magnet Weight
Frequency Response: 600 - 20k Hz
Sensitivity: 108dB SPL 1W/1M
8 Ohm Impedance
200 Watts Peak Power Handling
Fits Horns with 13/8" x 18 TPI Threads