Warm water of a data center won't change the temperature of the ocean at all.
Not the whole ocean, but as I said, it could affect the local environment.
But any way, what is the other option?
Did I say they shouldn't put warm water in the ocean?
What should be done, in all circumstances, is a study on the environmental impact. Such a thing may find (for example) that the original design releases the water in a secluded shallow bay where there is little circulation. As you said, a simple fix would be to make a longer pipe and release the water in deeper water, where there are stronger currents.
Environmentalism is not about doing nothing because well everything affects the environment. It is doing the optimal thing based on scientific evidence.