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.
Seirbhís Póilíní an Aerfoirt
Cór Póilíní an Airm
Rabhadh tugtha ag na póilíní d’imreoirí chumann CLG in Oirthear Bhéal Feirste a gcarranna a sheiceáil d’earraí amhrasacha
Chinn na póilíní nach raibh fianaise ar bith ar fáil go raibh éinne ag bagairt ionsaithe ar na hoibrithe custaim (i Latharna agus Béal Feirste)
Tugtar "póilíní" ar póilíní i dtíortha eile freisin:
Níl póilíní amhrasach níos mó faoi Christopher Hughes i dtaca le cás tragóideach Essex RTE
Fear maraithe ag na póilíní i Sasana i ndiaidh eachtra sáite Peig
Leath bealaigh ag an mbáicéara cloisim aláram carr póilíní i bhfad uaim… Tuairisc