Thanks to Squanto, the Pilgrims had a decent harvest of corn and squash (pumpkins mostly, I think) in the autumn of 1621. Also, thanks to Squanto, the friendly neighboring Wampanoags visited bearing venision. Indians likely outnumbered the English at this feast day.
It seems Turkey, roasted pumpkin, mussels, eels, cod, roasted or stewed venison, corn meal mush, and maybe lobster were on the menu: First Thanksgiving Meal.
There seem not to have been a lot of oysters nearby (maybe the Indians ate them all), but plenty of Cod. Ever heard of Cape Cod Turkey?
They had to learn to fish and how to plant. They had no milk (no livestock on Mayflower unless you count the 2 dogs), and any sugar or flour on board had been exhausted during the voyage. The Mayflower crew (30 men) returned home on starvation rations in Spring, 1621. Mayflower was an 80-foot leaky old cargo vessel which was barely suited for anything.