Can we count on the boat captain to snap a few pics?
I mean, yes maybe he can take the photos but he's not a photographer, he's a boat captain.
Also, it's nighttime during the fireworks cruise, so the photos may not be exactly what you're picturing in your head.
What I would suggest is this: propose earlier in the day, somewhere close to where there is a photopass photographer. They typically ask you to leave your bags near their feet. During that moment, whisper to the photographer that you're going to propose. They will be ready to capture you're special moment.
You can THEN surprise your new fiance with the fireworks cruise, just the two of you, to end the night and celebrate your futures together.
This being said, where is a good spot with a photopass photographer? Depends on where you want to propose! If you want magic kingdom, then there are photographers all the way up and down mainstreet. Best angle for the castle & minimum background people is between Casey's Corner & Plaza Inn. Sometimes there are also photographers on the bridge to Tomorrowland, and they take pictures with the castle in the background - this typically is less crowded (depending on time of day). Happily Ever After can be very romantic, so you may want to be in MK for the full effect.
There are fireworks cruises for EPCOT too. In EPCOT, there is typically a photopass photographer in front of the France Pavilion and it's a very pretty back drop. Morocco has beautiful mosaics, but photographers aren't there 100% of the time.
Also suggestion - do it early in the day. After I was proposed to (not in Disney) I was a big cry-y mess! If I was in Disney, I'd probably want it done early in the day so I can cry, go back to the hotel to freshen up, and then enjoy the rest of my day lol!