sometimes, they say, sh%t happens! I've had a breakdown, throttle cable came off a Yamaha o/board on our 4m tender in Lime Bay, it was bloody bumpy anyway and we limped back at tickover on the main and full chat on the Mariner 3.3 auxilliary. We were travelling around so I fixed it, I thought, and next outing was in Barmouth where it came completely apart, again, and by the time we got the Mariner going we'd drifted onto a bar and buggered the shear pin. Naturally, the shear pin spares, neatly taped to the tiller handle, were nowhere to be found when urgently wanted. It took son and I over half an hours paddling just to get to the nearest beach and get out and walk it back. I made a temp fix for the missing shear pins, and plenty of Araldite ensured the throttle would never again come off. since then we have covered many miles at sea, both in Wales and off the South Coast with no problems apart from motor clamps coming loose last weekend (both motors are chained on anyway) and causing the thing to vibrate so much we thought there was a mis-fire problem.
I service my motors regularly and painstakingly, but there is just no taking into account Murphy's Law of the sea which is similar to that on land but applies only when you've got outside harbour limits before kicking in!