) - I have no personal experience but I've read often that he is the guy to see for repairs to everything Olympus OM. Otherwise you could try it yourself (I've also read - but have no personal experience - that Canada Balsam is a suitable bonding agent for lens elements... but the fisheye is probably worth too much to be playing around with it like that!) >
" ... rear element looks like theres 'water' in there... "
I strongly thing, it may be "condensation" of the water from the air which stays inside the Lens. I would wait to a while, before of turning for a repair-man.
Place the Lens inside a paper-pouch, next an air-drier; to a couple of days.
How such a "water" shows, drop or haze ?
If there is an huge drop, force it to slide toward the borders of the glass; by turning and placing the Objectiv on the position due: the drop should slide away, hour after hour slowly.
After of that only, place the Lens next the air-drier; We want to avoid, the drop leaves any haze, once dried.