It seems to be impossible to get true flush-fitting opening hatches now: probably a consequence of the dreaded RCD. Several firms do "low profile" hatches which protrude about 1 inch, I quote the heights above deck from their catalogues.
Thanks Lynette - think I may have to construct them myself - they have to be waterproof & strong enough for 20 stones of diver & kit to jump on - plus must be flush & I need about 10 of them!