Sara Underwood and fiancÃ© Andy York held their marine-themed party at Brancaster beach in Norfolk.
Mr York dressed as a large blue squid and Miss Underwood was an octopus.
Some of the more bizarre moments were when an excited dog tried to chew her tentacles, and an 8ft cod attempted to eat a fish and chip supper, she said.
The couple, from Brigstock, Northamptonshire, invited guests from as far afield as Cornwall to the beach for their fancy dress celebrations.
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Miss Underwood admitted the sight of 50 sea creatures arriving by coach and taking over the beach turned heads, when they arrived on Sunday afternoon.
"The local dogs were really startled," she said.
The bride-to-be's parents have a second home in the area and she said she had fond memories of childhood holidays there.
Guests embraced the theme dressing up as mermaids - "who got very cold" - along with "seaweed", "seahorses" and a "deep-sea diver".
They ended the party at a local fish and chip restaurant where many guests, including the 8ft-tall cod, attempted to polish off their supper still dressed in full costume.
"When the giant cod was waddling around trying to eat cod, it was all a bit surreal," Miss Underwood said.
"We wanted to do something a bit different and I've always wanted to dress up as an octopus... but I woke up this morning thinking, did this happen?"
A bride-to-be fulfilled her ambition to "be an octopus" by dressing the part along with 50 other "sea creatures" at a joint stag and hen party.