I think in some scenarios with the correct planning it can be pulled off to an extent of good RP, not diving into the water dropping items and then collecting them later on, for example, i have taken a chase before and asked someone on radio to get a jetski and meet me in the canals, I have then managed to get onto the jetski and avoid custody that way. I have also tried this and failed being stopped (rightly so) before entering the water (on foot). Now as for driving a car at high speed off the pier, I can see a reason that losing a car in RP is better than going to jail for X amount of time in RP, and the damages that it causes to the car ect but I think the basis behind people doing this is that they know their car will be fine and they get away regardless and have their car back at the next storm.
I feel this is a catch 22, due to the dynamics of the server, cars that go into the water can be retrieved for the same price as one that has been left/stolen, this doesn't correlate to the damages, for example, a car that has been dunked into the water for several hours even minutes will more than likely be in other words fucked, it would need a new engine ect. An idea to combat this would be to increase the cost to retrieve out of the impound ( RP basis - Time spent getting it out of the water. Time spent repairing ect.)
Now I think its the same as someone who is downed by police and does "bleeding out, no pulse." its all about moderation, its not so much that the idea of driving of the pier is wrong, but if every time you get into a chase you do that, then you're not allowing the other party chance to RP. The same way that if you "no pulse" consistently when you get downed you're avoiding continuing the RP, possibly to avoid prison time, potential loss of cars, items.
I think the only issue that lies with this is in some cases it can provide for some decent RP. And in some cases it can provide for very little RP. It all comes down to choice of the player at hand, do they choose to attempt a different escape and potentially get caught, or do they go for the pier where they more than likely won't get caught.
I myself don't care for items and have numerous times as you are aware took one on the chin, but it actually creates for good RP.
I feel some people fail to understand that getting away is not the be all and end all, and to actually have fun.