It's similar to saying if one is necessary on a Mac. Technically there are viruses out there for android, but many are a challenge to find.
I would opt on the side of caution and pick one up, but don't break the bank to do so.
Generally speaking, reviews have found AV performance on android to be lacking - it's really something you would weigh. You've got your two options, you can go underwater without your anti-seabear underwear on, since it is unlikely you will ever meet such a creature, but if the possibility exists you may wish to do so anyway so as to be sure.