It could be a grey area. I've never heard, or seen of a situation where a tenant would be supplied or reimbursed a hose to keep lawns maintained. However, tenants have probably never thought of trying it It's a little like tv antennas. It's not the owners responsibility to supply one, but many do just to keep the tenant happy.
What does it say in the lease? Does it say to 'maintain lawns' or to 'keep lawns/gardens watered at all times'? If it doesn't say to actually water, then that would be your defense. Not sure of your water restrictions also, but you could also use that as well.