Not sure if I would call it one of the most important routine maintenance
jobs but if your engine overheats then it will quickly move to the top of the list.
How hard it would be will depend on your engine and how long it has been since the job was done. I recently cleaned out mine on a 30 year old Westerbeke
and it wasn't too bad. Considering the lack of recent maintenance when I bought the boat it had probably been many years (if ever) since it had been cleaned.
hoses to the exchanger were old and hard so I ended up having to cut them off. They needed replacing anyway so no loss. On my engine it was attached by a couple of brackets and hose clamps so after the hoses were off it removed easily. It has been over a year now so details are getting fuzzy but I don't remember any serious trouble removing the end caps to access the cooling
At that point it was just a matter of a little rinsing and poking with a long metal rod and various tools to clean out the deposits but it really wasn't blocked up to any degree and I probably could have gone for years without a problem.
If you can run your engine hard under full load and the temperature doesn't go up at all you are probably not in imminent danger
of burning up the engine. But if you have no idea how long it has been since the heat exchanger was cleaned and you have nothing else to do (yea right, you have a boat so when does that ever happen) then not a bad idea to take a peek.