Steve is right on.
There is no requirement or need to stick with Motorola branded antennas. Motorola doesn't make their own antennas anyway, they're relabeled from other brands. In all the radios I've installed I've never seen anything from Motorola that requires using their antennas.
Marine VHF antennas can vary greatly in quality. Some of the low end models are simply a stripped back piece of coax stuffed inside a fiberglass tube. They'll work, but they really are not worth the price and I wouldn't trust one in your application.
The better marine VHF antennas have actual metal rods inside and will stand up to a lot more abuse. They'll be more expensive, but they'll last you a lot longer.
After a few years in the USCG, I can tell you that most of our marine VHF stuff was commercial off the shelf, including the antennas. They were the high quality antennas, but they did just fine, and that was years of being beat hard on the Bering sea. Never saw one fail.
The other benefit to specific marine antennas is that they use the standard 1"x14 mounts. While I love NMO's, I'd be reluctant to use on in a marine environment, especially if it was mounted in a location that didn't provide easy access for frequent maintenance, like up on the mast head.
You'd be best to go with a known company and order the specific antenna you need, not just a generic marine VHF antenna. You can get suitable antennas for VHF, UHF and 700/800MHz, WiFi, cellular, etc. They use the standard 1 inch x 14 mounts common to the marine antennas and are specifically designed for use in marine (even salt water) environment.
Ideally, you'd use one of these, ordered the specific model for your frequency, they have versions that cover from 144MHz all the way up to 174MHz. Rated for 250 watts:
MS150U-XXX - COMTELCO Communication Products