VHF marine radios are designed for communications across water. Marine radios use frequencies specifically set aside for this purpose and are great for general communication or if an emergency arises.
There are generally two types of marine radios: handheld and mounted. Handheld models are more convenient, as they can be easily taken from one boat to another. Mounted models require a little more work (installing the radio and connecting an antenna) but typically they deliver more power.