If you figure our the wattage requirements for your 1.5 amp draw, that 300 watt inverter will work, but....you'd have to wire that thing into the boat system somehow.
Check for a 12v power supply for your cpap. Many of them have them available as an extra purchase item....If available, see if your houseboat will have a suitable 12v receptacle in your berth.
CPAP, like a lot of electronics, are DC inside, so that AC plug is another power supply that adds some to your power loss getting the thing to run...
That will at least get you using the boat native 12v system without inverter losses. The next thing is making sure the houseboat has enough amp hours for your stay...based on the 1.5 amp at 120v requirement, your CPAP will probably be drawing around 170-180 watts max (you will likely find it closer to 170 or less once you are powering from straight DC) which will end up being something like 14-15 amps...
The boat will need that X how many hours you sleep in amp hour capacity to run your CPAP. If you are sleeping 8 hrs a night you'll be using up 120 aH or so....On NWD my CPAP is a larger daily aH draw than our full sized Vitrifrigo refrigerator.
If the boat doesn't have that power, you might think of something so radical as taking a deep cycle battery with you and your inverter...I still suggest the 12V power supply for your boat, it really does help cut down some of the power requirements.
"There exist minds that think as well as you do, but differently."