I'm going to go against the grain of the other recommendations and disagree with them a bit. First a qualifying statement: They are all correct in a normal communications, especially on land, situation.
Here's why I disagree and actually think that the amplifier is a good idea for an emergency (although illegal): The U.S. Coast Guard runs a lot more than 25 watts, and they're the only stations that count when you need to be rescued. I have been 40 miles offshore in a 25 foot sport fishing boat when you can't make out other stations that are at port, but you CAN clearly hear the Coast Guard. Not only does the Coast Guard user higher power levels, they also must be using some type of antenna system with huge gain. Here is a link to an interesting thread from some folks that monitor the CG near me:
Southern California Coast Guard Transmitter Locations - The RadioReference.com Forums
My point is that the "who will be able to hear you" argument against the amplifier is moot. The Coast Guard will likely be able to hear you if you're that far out, even if noone else will. And they are the only ones that matter.
On a sport fishing vessel, there is only so much you can do to get the antenna high without it turning into a big structural hassle. Yagi's and rotors are completely impractical, not to mention that most rotators would not last in the saltwater air. I would challenge that anyone who recommends something like that has not spent much time on the ocean in a small boat and has not witnessed how much everything gets beat up and fails. Just the constant pounding of the boat against the sea surface while doing 25 knots will kill the yagi system the first time out, if the salt water didn't get it first. It's difficult enough to keep bait tank pumps, fish finder transducers, and all the necessary electrical stuff in working condition. A yagi system will never make it. The ocean is not kind to its visitors.
For antennas, the best thing you can do is to mount the highest gain marine antenna in whatever spot is provided for your antenna and call it good. Unfortunately, most of the marine antenna manufacturers are probably a little "generous" on their gain figures to really be able to know what you're getting. You don't have to worry about installing ground planes because ALL marine antennas are either 1/2 wave (the cheap ones you don't want) or some type of phased array over a 1/2 wave (most are these and are much taller). You want the latter as it will have higher gain. You should also inspect your coax, connectors, and antenna annually to replace anything that has become corroded by the salt air. Also make sure that the antenna can handle whatever power you're planning on putting into it. I also wouldn't mess with larger diameter coax on a 27 foot boat because your cable run is probably about 15 feet maximum (if that). There isn't enough line loss in 15 feet at VHF to worry about. Just use good quality RG-8x or similar.
I'm guessing that an open console fishing boat is probably fiberglass, since that's what most of them are. There are very few made that are aluminum of that size, and probably exactly zero that are still made out of wood.
BTW, I have communicated with the Coast Guard in a fiberglass fishing boat with a regular VHF Marine radio at about 40 miles out. Yesterday I was out on the water on the back side of Catalina Island, which is about 30 miles out. If I was much farther out than that, I would probably want a bit more power, too. The ocean is a big place and it is not your friend. Until you've been out far enough to where you can't see any land in a small boat, you don't truly understand the feeling of helplessness on the ocean.