There is only one species in the family Argyonetidae (water spider), Argyroneta aquatica. This spider occurs in Europe and parts of Asia in either slow-moving or still water. This spider spends most of its life under water hunting small aquatic creatures. The spider makes a submerged "diving bell" that is used for air, feeding and several other purposes. Argyroneta aquatica can be up to 5/8" in legspan. I personally do not keep any specimens as they do not occur in the U.S.
I catched one of these water dwelling spiders once in a small pond in Sweden one summer and kept it for awhile and it's pretty aggressive speices.
They make like a water bubble on their abdomen and swim around under water like it was nothing.
It's also a pretty sturdy built spider.
Have also nearly a kind "Usambara" attitude, meaning it will go for you if you come to near.