My personal tail of horror happened earlier this year when I had a new stern gland fitted. Everything looked cool when she was up on the slips but looks can be deceiving.
When we put her back in I decided to see how she was moving with her nice clean hull so I went for a quick spin around the bay near our mooring. When I got back to the mooring and killed the engine I thought I could hear an odd sound so went a looking. The gland had come completely off the shaft and water was streaming in. It was already within a inch or two of the floorboards (and we have quite a deep bilge). No way was the electric pump going to cope, although I managed to keep the level steady with the manual pump. Alone on the boat this left me in a no win situation cos if I stopped pumping to try and fix the problem we were sunk, literally. Things looked grim, particularly as the yard had already started hauling two other boats so running her back up was not an option. (I'd invoked Sod's Law of the Sea as our spare electric pump was off the boat for service.)
Ah but the gods of modern technology came riding to the rescue. Phoned Ms Wombat at the office on my mobile, she rang the mechanic , who threw a portable pump into his workboat and rushed out to our mooring. The extra pump took care of the water, he reattached the gland and we spent the rest of the afternoon beating the crap out of his offsider who had incorrectly attached the gland.
So, although I'd checked the bilges after we came off the slips the problem only reared it's ugly head went I put her into gear and opened up the throttle.
The obvious lessons learnt were
Always have at least one spare high capacity bilge pump, even if it is a portable.
Don't just check the bilges when your boat first goes back into the water , do it again when the engine is running and in gear.
Keep a change of underwear on the boat at all times.
Most importantly, heed the words of the late great Douglas Adams and "DONT PANIC".
A very chastened Wombat after that experience but it was almost worth it to hear Ms Wombat's reaction to "Now just shutup and listen. I don't have time to explain, Raven is sinking. Ring Alan and get him out here pronto."
Ah such fun.