In New Hampshire we have a few ponds and lakes particularly suseptible to the conditions you describe. What the State has done on it's ramps is made the below waterline either a "caged bundle of rocks" (they call it rip-rap construction) instead of concrete OR they have roughed up the concrete to such a degree that there is always a sharp edge to grab.
I like Ricks idea of the mat that rolls up out of the water but I have stood on wet rubber before and it can be like ice. Maybe an indoor/outdoor carpet?
I think you do have to go with something that doesn't stay in the water constantly or it will have to be treated for slime inhibition.
The nice part about being a pessimist is that you are constantly being either proven right or pleasantly surprised. - George Will
"It's not that liberals aren't smart, it's just that so much of what they know isn't so" -Ronald Reagan
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