Finding the best marine speakers isn't hard if you know what to look for. There aren't as many manufacturers for marine speakers as there are for regular speakers, which limits your options, but at the same time, also makes choosing a speaker easier. Beyond the usual brands - JBL, Boss, etc. - there are also several other companies like Polk Audio, Pyle, etc. that you should consider.
First, a few things to keep in mind when looking for the best marine speakers:
1. Don't use your car speakers.
I know it sounds ridiculously obvious, but you will be surprised at the number of people who just plug in their car speakers into their boats. These speakers aren't meant to withstand water. Their cones are made of paper or other light weight materials susceptible to water damage. The best marine speakers are much more durable and have a high degree of water resistance. Some are even completely waterproof - an important consideration since the speakers will have to brave the elements on a boat.
2. You will have to use an amplifier.
A boat going at a high speed can be incredibly noisy. You will have to turn up the volume on your audio system all the way up to even hear anything. At such high volume, even the best marine speakers will struggle for clarity. To counter this problem, you will have to use an amplifier with your marine speakers. The amplifier will increase the strength and clarity of the signal, and help you get the best out of your equipment.
3. Figure out where you will install the speakers.
It has happened to me more than once - I've bought myself a pair of marine speakers only to find that they won't fit into my boat. Measure the speaker holes carefully before making a purchase decision. You don't want to spend 0 and countless hours scouring for the best marine speakers only to find that they won't even fit into your boat.
4. Read reviews online.
Although I assume that you follow this simple advice before buying any electronic equipment, it's never a bad thing to remind you again. I always make sure to check out actual customer reviews of my chosen electronic equipment before plopping down my money. Amazon.com is a great place to start. You can also search for "[product name] reviews" on Google to see what actual users feel about the speakers.