Hi ff and all,
There's an old saying, 90% of the station is on the roof. In other words the antenna and where it's located is the key, keep that in mind and you're that much ahead of the game. Receiving the Coast Guard boat stations is only half of it, the other half and the difficult one is receiving the boats. Half a conversation is half a conversation.
You're getting the idea with the ham antenna but you don't need the 70cM side naturally, a monobander will have much higher gain and that's what you need. A base antenna is easily assembled and disassembled, go for a temporary portable installation if at all possible. For what it's worth the Cushcraft ARX-2 can be tuned for any VHF band segment and a resonant antenna having the highest gain naturally is an advantage over one tuned for 2M. The chart tells you the dimensions, the ring matches the impedance and is of lesser importance. That can be tuned for 2M and left there, you then have a multi purpose antenna.
BTW, there's more to the marine band than the USCG. Glimmer Glass has me wondering why he annoys the bridge operators requesting several openings every day, he's a royal pain in the butt. (;->)