Re: new boat
An 18' boat with a 115 should be pretty fast,40-45 anyhow, I would suspect the engine isn't running quite right so it is only making a fraction of the power it should .
If the boat is rated for a 150 then ya, if not then don't do it.
Boats are rated for a specific horsepower, if you exceed that you cna do irreparable damage to the boat at the very least and possibly hurt or kill someone in the deal too.
There should be a capacity plate somewhere by the console that has load and HP info