Make it so that the activator button is the actual rod item, this will keep your ability menu free for other abilities.
You then need to check for water, this probably will need JASS but you could also litter the coasts with helper units which are invisible (if near one then you can fish, else it says you are too far from water).
Fishing itself is hardly complex. You use random numbers and maths to get the chances for certain outcomes, and then via multiple selection you can check if those chances occur (if so do the appropiate actions, otherwise move on to next check).