I've seen more boats have problems due to water cooling issues, and don't really see the need to risk it on a boat that is already having some issues. If the engine continues to run, you can run as many large electric bilge pumps
as you need...but as soon as the engine fails... you're down to your batteries and whatever manual bilge pumps
It is really a personal choice, but I think that having a small bilge pump
for the day-to-day stuff, and one or two large bilge pumps
for emergency use, that have a higher float switch than the day-to-day pump does makes far more sense. As long as you have diesel and can keep the motor running, you'll have electricity to keep the pumps going. If you use the raw-water cooling as part of the pumping solution and it gets blocked with crud... then you're down a pump, but also have lost most of your electrical generation capacity, and possibly the ability to move your boat toward safety...since the diesel is also powering the prop...
A small hole in the bottom of a boat, say 3' below the waterline because of a grounding, even if it is only an 2" in diameter will let in 136 GPM
... Given that most bilge pumps
are rated at GPH, not GPM, the largest 12V bilge pump
I've been able to spec out rates at 3000 GPH, or 50GPM, which would require three of them to stay ahead of a small 2" hole. That is an optimistic estimate as well, since the pumps are rated at 0' of head, and in this case, we'd have at least 3' of head, to get the water out of the bilge and up past the waterline. I don't believe a raw water cooling system runs at 50 GPM, given the small diameter of most of the tubing and passages in it.