ehhh, high tide?
Let's not assume a tsunami. Suppose we have to raise the average water level by 3 miles.
Using the figure from here, the Earth radius is 3,959. Take that as center to sea-level.
Volume of the Earth, cubic miles:
V = 4/3pi(r^^3),
2.599234 x 10^^11 cubic miles.
Same thing with radius plus 3 more miles of water.
2.6051457 x 10^^11 cubic miles.
The difference: 591,331,720 cubic miles of water.
Present estimates of the total Earth's water: 326 million cubic miles.
Amount of water then / amount of water now = 1.8