How to clamp a hose below waterline when there is not enough room for two clamps?

[bnort3]

I would add a high volume bilge pump to your engine room with a high water alarm near your electric panel. Then, if a clamp fails and springs a leak, eventually your bilge alarm will go off. As others have noted if the clamp fails, the hose, if fitted properly, won’t just slip off the hose barb. It may still leak but probably not enough to overwhelm the bilge pump.