So ... how about having a tab similar to the economics map in the ledger stuff where you 'buy' gunboats as you do with factories and therefore there is no need to change the map graphics - the 'level' of interdiction whatever could be reported on tooltips for the river sections concerned and then the gradiated interdiction chances could be bought into play.
You essentially invest in a blockading force or have active individual vessels for other roles - or do away with the indiviual vessels totally (which I think would be a better option) and just have a more abstracted brown water naval function.
Some form of calculations could then be written/made to erode cohesion of this investment incorporating it with river transport values to work out available transport for power projection missions etc.
May be too complicated mind
