All marine VHF radios must be licensed. A marine radio station must also have a call-sign allocated to the vessel it is installed in. Shore based stations can also be licensed but only where they are involved in marine based activities e.g Yacht Clubs, Marinas, Harbour Masters, Rescue Services etc.
In the UK ships radio licences can be obtained from Ofcom. They are free for non-commercial users.
The user must also have a Marine VHF Radio operators certificate. This requires taking an exam. In the UK contact the Royal Yachting Association for a copy of the syllabus and local examiners.
More information about the licensing rules, courses and exams can be found here.
The ICOM ICM55 is a good radio set and can still be used in an existing (marine) installation but it does not meet the standards for new installations as it does not have DSC (digital selective calling) capabilities - which all new fixed sets must now have. It is therefore very unlikely that you will be able to get a licence to install one.
To get it working you will need a proper marine band VHF aerial which operates in the frequency range 156-MHz. They usually come with cable attached but if a longer cable is needed, or the aerial is not cabled in the first place, use flexible 75 ohm co-axial cable with a copper core and braid/foil screen. Use proper aerial connectors (type F or BC are most commonly used and can be obtained from most good electronic component suppliers more cheaply than from a boat chandlers. Do not use push-fit TV aerial plugs)
Citizens Band radio works on the much lower frequency MHz band and therefore a CB aerial will not work with a marine VHF radio set.
Please see this Wikipedia article for more information http://en.wikipedia.org/wiki/Marine_VHF_radio