Urinator means "diver." You can look it up in the OLD if you want, you'll get diver. Since it's easier to find for most people than the OLD, here's Lewis & Short's entry on urinator: "I.a diver, Varr. L. L. 5, § 126 Müll.; Liv. 44, 10; Dig. 14, 2, 4; Inscr. Orell. 4115." It means diver.

But what is this collegium urinatorum that you're talking about? There were corpora piscatorum et urinatorum, "[legal] bodies of fishermen and divers," at Ostia and elsewhere that we know about from multiple inscriptions (CIL 14.303, mentioning a patron "corporum mensorum frumentariorum et urinatorum"; CIL 6.1872, mentioning a "corporis piscatorum et urinatorum totius alv(ei) Tiber(is)"; CIL 6.29702, "[corpus piscator]um urinatorum"; CIL 6.40638, "[corpus piscatorum] et urinatorum"). An additional inscription (AE 1982, 131) mentions a "corpus urinatorum Ostiensium," without the addition of the fishermen. But that's not the same thing as a collegium necessarily. These inscriptions date to the later part of the empire, by which time legal corpora appear frequently as groups set up to streamline particular functions. Sirks ("Qui annonae Urbis serviunt. De juridische regelingen in het Romeinse keiserrijk inzake her vervoer van onus fiscale, met name voor de annona, over zee en over de Tiber") reasonably conjectured that many corpora in the later empire were set up specifically for the public interest. Many can certainly be linked to the cura annonae: the corpus pistorum, corpus naviculariorum, corpus lenunculariorum, and corpus caudicariorum seu piscatorum certainly were all involved in shipping grain from the provinces, hauling it from port up the Tiber, and then baking it into bread. The corpus piscatorum et urinatorum seems easily to fall into this same group, and Marzano (Harvesting the sea: the exploitation of marine resources in the Roman Mediterranean) suggests on the basis of the existence of the "[legal] body of fishermen and divers of the whole Tiber riverbed" mentioned by CIL 6.1872 that the divers at Ostia and along the Tiber were mainly involved in the retrieval of cargo, particularly grain, that had fallen to the bottom during the journey upstream. The fishermen present an issue however--did they perhaps hire out barges, or are they maybe fishers in name only along the Tiber? Marzano has more to say on that subject, but in any case there's little reason to generalize the work of divers. Those working along the Tiber might well have had different organization and duties than elsewhere, though no other professional organizations of divers are attested anywhere else in Italy--perhaps this was a technical term?

Is this a collegium? Legal corpora of this type are pretty much without exception connected somehow to collegia, though it's sometimes hard to say how. For example, we know of multiple collegia pistorum in the city across many periods. Were all collegia of bakers by the later empire grouped into a unitary legal corpus for the purpose of the cura annonae? Nevertheless, while legal corpora of this type and collegia never seem far apart, there is no direct evidence that a collegium urinatorum ever existed under that name, or even that there were collegia piscatorum et urinatorum. CIL 6.1872 mentions a quinquennalis for the corpus, an elected office typical of professional collegia in the Principate, and another inscription, CIL 6.29700, also refers to "piscat(ori) urinat(ori) q(uin)q(uennali)," though it does not technically specifically designate the piscator urinator as descriptive of either a collegium or a corpus. There is very good reason to conjecture that there were at least two collegia piscatorum et urinatorum at Ostia and along the Tiber, possibly grouped into a single legal corpus or possibly two corpora made up of several collegia at sites in the area. But we do not directly know this. Modern scholars do not refer to the organization(s) mentioned on these inscriptions as collegia, but instead as corpora, which is what the inscriptions say. Wikipedia cites some 1898 guy, presumably for both the attribution of the term collegium and the translation (I'm not really sure, the page doesn't explain and the citation doesn't link anywhere)? The earliest use--in fact, the only one I could easily find after some quick Googling that doesn't go back to the Wikipedia article--of collegium to describe the urinatores (who, to my knowledge, are only mentioned independent of the fishermen in two inscriptions, AE 1982, 131 and CIL 14.303, in the latter of which they are lumped in with several other corpora) is in The Encyclopedia of Social Reform edited by William Bliss in 1897. This publication, which I think it's unnecessary to say does not represent current scholarship, makes a number of missteps regarding collegia. Many are simply because in the 19th century almost nothing was known about professional collegia, but others are simple and hard to excuse. The encyclopedia, for example, mistranslates urinatores as "devils" somehow, and calls the collegia pistorum (it uses the singular for all collegia, inexplicably), a "union" of "millers," though the term is ubiquitously accepted by modern scholars to describe bakers in this and almost all contexts (Plautus and a few others use it to mean miller).