The OP seems to like quite specific info, so here is a specific answer to the OP question, "When should I get dive insurance?"
The answer is: before you need it.
OK, so when do you need it? (1) to go on some trips, so this is an easy one....if the trip says you need it, you then can get it. (2) you need it before your next dive accident, but if you are like most of us, you don't really know when that will happen. But you can be assured it will NOT happen unless you go diving. So, you need the dive accident insurance before you next diving trip.
The answer is: before you need it.
OK, so when do you need it? (1) to go on some trips, so this is an easy one....if the trip says you need it, you then can get it. (2) you need it before your next dive accident, but if you are like most of us, you don't really know when that will happen. But you can be assured it will NOT happen unless you go diving. So, you need the dive accident insurance before you next diving trip.