We occasionally train to shoot from the stern, bow, top of the deck house, flying bridge and other parts of a boat in the water at both towed and stationary targets.
We use rifle, shotgun and handguns.  For on-board targets we use simunitions or sometimes paintball guns.
Anybody else training this way?
What do you use?

I used to use a UH-1C, but that was 32 years ago. Firing from a moving car (safely in a bumpy field) was nearly impossible to get any positive results. Just a personal experiment that was abandoned.

With a M-60 by any chance?
I've had the same experience with the moving car, but it is a little easier on a flat paved surface and out of a van thru the open sliding cargo door while in the middle area is even easier.
On board a boat, if you're able to stand without having to brace against part of the structure, you can flex your knees and get into a rhythm.  But you quickly learn to appreciate a lanyard and not to do speed reloads without catching your expended magazine!
I've seen several expensive pistol mags drop into deep water, never to be recovered.

M-60 and .50 cal., the 20MM we had on hand was not approved for the airframe. I was driving and shooting in a bumpy field, not just riding "shotgun". Very hard to do.