Before I start this I would like to say that I hope that no flaming begins here cause my @ss doesn't take heat normally, and thus isn't tempered.
Ok, Yes there are dolphins and wales in starfleet. See the blue prints of the USS Enterprise-D. They went on sale in the late 1990's.
Furthermore, I speculate that even though their sentience has been argued for a long time now . . . actual renewed contact and recruitment probably didn't start in any amount until after the Whale Probe Incident, of which Star Trek IV was about. Thus dolphins, whales, and their like haven't been in starfleet in large numbers @ any point until now.
Besides that, we have no canon record that the senders of aformention probed have been contacted, or attempted to contact us, since the incident. Therefore carrying whale or dolphin crew members would be in the best interest for Starfleet, not only for navigation of communication purposes, but just in case a starship was to meet that race again as not to be neutralized.
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"See Everything; overlook a great deal; correct a little." -Pope John XXIII
I didn't mean to start a debate on whether or not Dolphins are sentient, just that I thought it was pretty wierd, and never really saw or heard anything about it until now.