Bought these for a vacation.  The back strap is useless, no matter how tight you make it.  It's not attached to the other strap, so it falls down and becomes even more loose, plus there is elastic stretching it even more..  Maybe after some tinkering these would work, but they're not too easy to walk in without the back strap.  After taking about 3 steps, the back strap fell down and would end up under the sandal, where I stepped on them. The grip on the bottom is good though, not slippery even in water.