There are a few key considerations here with the main ones being the size of your boat and also your distance away from the coast. You also have to consider options such as whether you have got the battery power to run a fixed mount radio.
If you are going to be staying within five miles of the shore on day trips then having a handheld VHF radio is going to be perfectly fine for your needs. You won’t have to worry about the battery life and you will be able to get in touch with the coastguard.
If you are spending longer out on the water and further away from the shore, then it’s best to get a fixed mount radio. Also, fixed-mount radios tend to have more functions along with their increased power. If you can, it’s probably best to get a fixed mount radio installed as this would give you more versatility.