I think my previous post got missed, but this isn't really possible. The soft safe and soft kill zones are cumulative if you do the behavior you describe. Just leaving the zone doesn't reset the timer unless you actually leave it for a while. Jumping up and down, touching the soft kill on the down, wouldn't reset the timer.
Interesting. Well then, my previous point about just using a soft kill zone instead of a hard kill zone to give people a few grace seconds still stands, since the whole jumping to avoid the limit issue was brought up as a potential way of beating that grace period. Still, given short water crossways, you could theoretically build gaps in the islands or structures just big enough to be crossed in under ten seconds at a regular walk. It would make certain walkways interesting, because no one could camp on the walkway--they've got 10 seconds there in the water, or they're done. It would be interesting to add the dynamic of certain areas of the map where survival is the priority, as opposed to kills or the objective. Do you risk being in for ten seconds to kill the guy behind you, or do you risk being shot up while you try to make your way to safety? It would be quite the interesting setup.
Meh... I recall a post about this some time ago here on ForgeHub. It may have been somewhere in the reach forge discussion forum, before the game was released.
I think a while before Reach was released we were discussing doing it with a teleporter, rather than a kill zone. You could set the teleporter up to teleport players in to a kill zone,but not vehicles. You could even have tanks teleport as well so you could have 2 "islands" which can be reached using light vehicles and then defensive wraiths on each island acting as a mobile turret. Of course, if you do that then there's no need to even set the map on the water, you could set a map up which uses shield doors over an lowered portion of the map. You could give the map an industrial feel with pools of toxic sludge rather than waving water. Just ideas.