As you look at different SWR meters, look carefully at the specs to be sure they are suitable for marine
VHF use. They key specs are the power
range and frequency range. I made the mistake of buying
a Radio Shack SWR because it was convenient to get, but it only works up to 30Mhz which is well below the marine
band. It was useless and I had to return it.
When looking for a replacement, I found many units that only worked up to 20W, and you really need 25-30W. Some had two ranges, 0-20W and then up to 300W, neither of which are suitable for a 25W radio.
Here's the one I finally bought Ham Radio Outlet | MFJ MFJ-822 | HF/VHF 300W X-NEEDLE SWR/WATT METER
. Don't forget you'll need a short jumper cable as well. This helped me track down a very perplexing radio problem.