Vattnet. Vatten is a neuter word, but the unstressed -en gets reduced to -n with -et. It happens with -er as well (väder, vädret).
It isn't silent, but it melts together with the D to form a special sound that is sometimes called retroflex. This happens in Standard Swedish whenever an R appears before an N, T, D, L or S (with S it sounds a bit like a 'sh' sound).
Vatten sounds like "vattan". But actually, e is simply an e, isn't it? I have read that in general the Swedish e does not differ from the German e in pronounciation unless it is next to an r.