The first thing you will need to do is confirm that it is not just low speed but also low rpms, as this will pretty much eliminate a impeller problem and point towards engine/fuel.
You mentioned starts fine out of water, are you saying that in water it is harder starting as well?
Did you do anything right before this problem such as rolled it, new fuel, etc?
Have you tried replacing the spark plugs and checking compression? If not this is the next place to start.
Being a 2015 you may be limited in what you can and or want to do as it is possibly still under the factory warranty.
It may also require the scan tool in order to check systems and for codes.
You have it as a 260 however I would need the full model as well, rxp, rxt, rxt-x, gtx, etc.
There are other things to check such as the supercharger and no restrictions, oil level, good fuel, fuel pressure, not overheating, checking for misfires such as with a timing light when under a load.