Hi Dave, Welcome.

I live in an urban seaside area of dyked suburb in the Pacific Northwest. My lot is 30% under water during the rainy season due to lack of drainage. I'm wondering whether a forest food garden would survive/thrive with such abundant moisture?

Terry, if you design well, it should survive. Many good food crops are floodplain forest trees: pecan, pawpaw, elderberry, blueberry, bur oak, and others can take flooding of one sort or another. A key variable is how long the water sits after rain--if its too long it can various species--that varies species by species, how long they can stand submersion. You can also create earthworks or hugelkultur beds to help raise the elevation of the crowns of the plants above flood level or make it so they aren't submerged as long. Find out the native plants that grow under such conditions in you area and then figure out which ones are useful and which ones you can use to make ecological analogs, substituting a similar species for the one that's native that you cannot use. Do beware of contaminants in the flood waters though. What's up stream in the watershed? You may want to design to filter or treat the flood waters as it enters yoru property with a riparian buffer or filter bed of thickly growing plants to reduce pollutants coming onto your property . . .

Good luck!

d