Concreting under water is really an specialty. If it is just the watertable, the only case I have personally found, the alternative I have always seen here, even for riverside locations that need hube pumping, is to lower the watertable and then proceed in dry.
Even for bridges it is made so with the help of pile sheet wall plus temporary islands etc.
Another alternative that forfeits much of the problematic cases is the use of steel sheated in-filled or not piles. This way you can pass to soil the loads efficiently and in a far more controlled way.
If you still want concrete under true water, normally at least these things I would consider
first if possible do what above (forfeit concreting under water)
if not, see if the current will disrupt the placed concrete; you may need to build underwater some protective contraption to ensure that you won't be merely throwing concrete to one current able to wholly disperse it.
If it is a footer, it normally never will be one footing on the surface of the submerged ground. A cavity should be extant, and big enough, yes, to take unto account the detrimental effects of washing, the submerged ground strength and spread of the concrete.
Also, this operation if needs diving should be made by experts in the field and in compliance of the safety measures required.
Concreting underwater uses normally concreting funnels, to ensure that the concrete dumped in the funnel when it comes to the bottom will meet surrounding concrete around; this will diminish washing.