What are you expectations ?  What do you want to do with the boat ?  Fish, ski, tube, just family cruising ? 

How many people will be in the boat ?

Ski will take the most hp. 

I had a 16 ft Larson with a 120 hp maybe it was 140 hp, inline 6.  It was an inboard motor.  20 years ago.  I would take up to 6 people but I could hardly get one skier up.  Even with 4 people in the boat it was a struggle. Just 2-3 people was about right. 

You can change the pitch of the prop to give more speed and less beginning pull or just the other way, less speed and more beginning pull.  This helps to a certain degree. 

Now just pulling them on a tube does not require much hp. 

2 years ago I got back into boating.  I go about 3-4 weekends per year.  I bought a 16 ft 1975 Ebco with a 150 hp 2 stroke out board.  I have more than enough power, but the price was right for the amount of use I use it.  I usually take 6 people in the boat, but my 14 year old girls and their friends prob. only weigh 100 # ea and I am 180 #.  You get the idea.