Rayquaza is only the first evolution
All the oceanic Pokemon we' seen have
been found in shallow waters, never more
than a few mnutes from the coast.
Even , where several cities
and the Pokemon League were
islands, was ultimately fenced in.
And given ht we' re crushed to death
by the atmospheric pressure when we use
Dive, it' s safe to say we don' t go very far.
There are probably countless varieties of
Pokemon we' never seen before out on
the open ocean, past the rocks that close
us in, where our Pokemon can' t take us.