Boat Fun

I acquired an inflatable boat for diving and fishing and got my first run out on the river yesterday. The boat is a 14ft Zodiac Futura which needed a lot of TLC, infact most people doubted it would ever hold presure. The engine is a 1981 Johnson 25HP Thunderbolt which I changed the gearbox oil and plugs a few weeks ago. Against better judgement I left the water pump alone as it was pushing plenty of water. The boat inflated and stayed inflated and the engine started first pull and ran smoothly, we pulled off the slipway with me still patting myself on the back. They say pride comes before a fall and sure enough 5 minutes up the river the water tell tale started belching steam. I can only imagine that the water pump stopped effectively doing its job and the engine over heated and boiled off the remaining cooling water. Does anyone have any inside information? As an old boss of mine said 'all boats sink' and as someone else said 'boats are just holes in the water that you throw money into' Anyway tommorow at work will mostly be spent ordering a new impeller and gaskets.