Yes, B&M made some marinized transmissions for a couple of years.
If you want a powerglide, find a "shorty" from some of the rail dragster
guys at a good price. They made two versions from chevy; long and short
tailshaft, but the shorty drag style has an even shorter output shaft and
a flat bearing plate that bolts to the transmission case.
They do take a lot of abuse. Don't need the converter, just need to
get a marine adapter with the internal splined shaft that will bolt up to
the flex plate. If you are using it with an outdrive, just set the shifter up
for neutral, low and high. Drive will take care of reverse and well, the park
setting..... Also, there were two factory gear ratios and I know there
are many more custom ratios. Just loop the cooler lines or run them
thru a small heat exchanger if the boat is going to see a lot load.
A-1 Transmissions (Marv Ripes) was the guru at one time in So Cal.
Good Luck,
OR