I saw a special on this a very very long time ago so some specs are sketchy. but anyway there was a group of fisherman that fished with nets, like just letting them out and what not, regardless one day they were pulling in their nets and came up with a "Loch Ness Monster" carcass. however they found this carcass in the ocean, not loch ness.
But to tie this all together, there has been documentation of sitings of the "Monster", and those sitings have been almost identical to the carcass.
Therefore yes i believe in it, however i think it was/is an extinct animal. and it's ecosystem is a little large then Loch Ness Scotland.