Everything needs to be kept cold for the tempura. Put the flour in the freezer ahead of time. Chill the water. Keep the seafood cold.
Prepare oysters, make sure they're free of shell and grit, keep cool til ready for cooking
Slice mackerel and keep cool til ready for deep frying
Heat oil to medium hot
While oil is heating make the tempura batter
Mix chilled water with kewpie, mix well
Add flour to water & kewpie solution and just combine, do not over mix
Once the oil is hot, cook the oysters and mackerel in batches so as to not overcrowd the frying oil or cool it too much
Coat a few oysters &/or mackerel in the batter then place gently into the hot oil
Allow the oysters/fish to cook for a few minutes. When the item floats and is golden in colour remove it and drain on a paper towel
Repeat with the remaining oysters and mackerel
Once all the oysters are cooked dab a small amount of wasabi on each oyster
Serve oysters and mackerel with a tempura dipping sauce