Those monsters were always coming out from the water meaning that it is 1 meter deep or so but when Riddick faces the biggest one of them all, he slides on his knees on the water all the way to the other side :D This is of course not considering the perfect ground enabling him to slide very smoothly :))
yeah, water probably wasnt deep. mud under it made sliding possible. but when it happened it made me think that couldnt he just run over that mud really quickly and escape before monsters get out?
also one other thing, although its probably nit picking but still. there are variations of monsters in Riddics world. buried ones, flying ones. so how they all survive? how they feed? isnt ecosystem and chain of food needed to sustain such large creatures as those monsters? deserted planet with only dog like animals, those monsters buried underground and fish like creatures being only habitants doesnt seem very realistic scenario