... Much has been said in praise of a new etymology of Poseidon.  Fick proposed to connect it with οἰδέω, rarely οἰδάω, to swell, οἶδμα, the swelling of the sea, or the sea itself.  With the preposition πός for ποτί, πος-ειδ-άων is supposed to have meant the swell.  The transition of προτί to πρός is intelligible enough, nor can it be doubted that the Doric ποτί takes the place of προτί.  But it should be remembered that in ancient Doric—(and the name of Potîdas is supposed to be old)—the final ι before a vowel is not elided, and if Boeckh admitted it once in Pindar, O. vii, 90, this would probably not be regarded as a valid excuse for Potîdas.  Secondly, there is, as far as I know, no other case where πός stands as a preposition before a verb.  Then there is the real difficulty of the short ι in ποσῐδήιον which cannot be separated from ποσειδῶν.   I mention all this not as in my opinion fatal to the etymology of Poseidon, but only as showing how easy it is to start minute objections to almost any mythological etymology, and how much more difficult to remove them, or to account for them.  What makes me hesitate much more before accepting the etymology of Poseidon as the On-sweller is the purely descriptive character of the name of this son of Kronos, though until a better etymology is suggested, which I shall hope to do further on, we may perhaps be allowed under reserve to retain it.  I see, however, that Brugmann, though giving all the dialectic varieties of the name, does not endorse Fick's etymology.

Anthon says:

The etymology of the names Poseidon and Neptunus is doubtful.  Poseidon is written in Doric Greek Poteidan (Ποτειδᾱν), of which we have another example in the name of Potidæa, written Poteidaia (Ποτειδαία) in the inscription, now in the British Museum, on those Athenians who fell before this city.  The name, according to some writers, contains the same root in the first syllable as we find in ποτός and ποταμός; and has the same reference, in all likelihood, to water and fluidity. (Contributions to the Science of Mythology, Müller, 1897).