Police in places with water often have boats, since crimes, and evidence of crimes, often end up in the water. There's at least one police officer here (not Ireland) dedicated to diving our rivers looking for evidence, bodies, etc. Or think Miami Vice. Tá na póilíní san uisce.
I've never heard of or come across "Garda" boats. I would've thought any sort of marine policing of would be the responsibility of the Navy. However, one time while exploring at a harbour, I came across a boat which was marked as a police boat. I don't think it had the same logo that our Gardaí have but they were a type of police, mainly dealing with customs issues such as finding drugs and they were either plain clothed or had a completely different uniform I can't remember. As mwasson points out above, incidents can happen in rivers etc and I've heard in the news of the "Garda diving squad" searching if a body was being looked for in a canal or river so they could well have boats too.
According to Wikipedia, there are Harbour Police in Dublin Harbour and Dún Laoghaire Harbour. They are essentially private police forces set up under statute, though the Wikipedia article suggests that the work has essentially been subcontracted out to private security firms in Dublin Port over the last few years. (And they are explicitly excluded from the muzzle requirements in the Control of Dogs regulations, believe it or not).
And then there's this.
"Tá bád ag na póilíní " would never ever be read as "A boat has the police", never mind "easily read".
The structure for translating "(somebody) has (something)" is "tá (something) ag (somebody)" - as "bád" is a (something) and "na póilíní" is (a somebody), the sentence means "the police have a boat".
When the subject of the verb is a (somebody) rather than a (something), then "ag" perfoms a different function, and is used with the verbal noun to express the progressive present. If "bád" was a verbal noun (it's not!), then "tá na póilíní ag bád" would mean "the police are boating".
That makes sense in general but some old sailors consider them to be somebody. I've run into this same question of word order when people talk about a cat, some think of them as a somebody and others as a something.
Next question, does Irish give a hint about this subjective shift when you convert "a boat" to "the boat"?
For instance, what do you say if you want to say something less ridiculous than "A boat has the police", say if you wanted to say "The boat has sailors"? Would it be "Tá an bad ag mairnéalach" or something else? Or if it is a particular boat like The Minnow, would "Ta An Bodairlín ag Skipper, Gilligan, agus na paisinéirí?"
You misunderstand; it's not about what is considered a "somebody" versus a "something", it's about the word order. Tá bád ag na póilíní will always be understood as "The police have a boat" because the word order is Tá Y ag X = "X has Y." The first thing is possessed by the second thing. Using this construction, "The boat has sailors" would be Tá mairnéalaigh ag an bád because, again, it's about word order. "A boat" = bád; "the boat" = an bád.
Let me rephrase my continuing confusion with this. I've been acting under the impression that for simple sentences Irish uses a VSO sentence structure except for some sentences being VOS. I've been adding sentences such as this one to my mental VOS list one by one when they occur. Is there a list somewhere of these exceptions or are they to be encountered one by one?
Gramadach na Gaeilge goes pretty deeply into the various kinds of complex clauses that Gaeilge allows.
I don't think this counts as a VOS sentence, though: the subject, "bád", is still second. You can argue whether "bád" is the semantic topic but it's definitely the syntactical subject.
It might help to consider the "tá X ag Y" construction to not be parallel to the English verb "to have", but instead to a non-existent "to be-had-by" verb (which you can approximate by using the passive voice). So "A boat is-had-by the police."
I'll admit, while I understand this, I still screw it up a lot--it's not exactly second nature to me, and I'm likely to mess it up if I'm tired. I do find it's a little easier if the possessor is denoted by a preposition contraction like agam, aici, etc.