You don't need all the units in your whaleboat maxed for the first missions that come out.
~1/3 to 1/2 of my whaleboat crews on DMM are under lv40 (unevolved) and I can do the first 4 levels of each of the first stage type.
You will need them maxed eventually though.
I have the units for a slice boat definitely, of the last 6 units I got 5 were slice types.
Going to take so long to level them.