3km from wi-fi is unlikely, although possible, but not on a boat.

From 5 watts of output on a marine hand held radio you can expect about 5/6 miles line of sight, right. The output of a wifi device for a computer, whether it's internal or external is about 50mw, also it's microwave to so doesn't propagate well at all.

The only way you can get that kind of range from wifi is to use an extremely directional antenna. This is to focus the 50mof power into a fine beam rather than radiating it everywhere. the problem with this is you must maintain line of sight, and with an antenna as directional as the kind required to achieve that sort of range you would struggle very much to keep the connection as your boat rolls around.

The best solution for internet access is probably going to be one of these phone dongles. They're about 15 quid a month.