First off I have just purchased a 15x54 semi-v jon boat. I am needing a motor. My father in law has a new briggs and stratton 5hp outboard. Would this be plenty for me to get around small to medium lakes and watersheds?

It will move your boat, but that's all.  I have a 1548 jon, and mine won't plane on 9.9 hp.  It's a nice barge on 9.9 hp.   I've since traded it in on a 15 hp.  I haven't taken delivery of the 15 hp yet, but I am not expecting any miracles from it, either.  

If you need any sort of speed, I'd consider a 25 hp.  I'm sticking with 15 hp because I have hp restrictions on some lakes.  Otherwise, I'd go to 25 hp, too.

On the upside, the B&S has a good reputation for being durable.  Since it's air-cooled, you don't have to worry about a water pump, or running in dirty water.  

Will it make the boat go?  yes.

Will you be happy with the speed in which the boat goes?  probably not.

One of my boats is a 12 foot vee, with a 4 horse Evinrude, and it will not plane, and is quite a dog, especially with 2 people.  But, it is MINE that I paid, for, and seeing as I am 14, that means quite a bit.  

Also, someone said that the B&S motor is reliable, and I haven't heard anyone say they aren't.  But, being air cooled, you have in effect, placed a lawnmower engine, on a lower unit.  On land, lawn mowers are pretty loud.  Think about how much better water carries sound, than land.  Also, I think it is a single cylinder motor.  That thing is going to vibrate all to heck and back.

First off I have just purchased a 15x54 semi-v jon boat. I am needing a motor. My father in law has a new briggs and stratton 5hp outboard. Would this be plenty for me to get around small to medium lakes and watersheds?

Sure that 5hp B&S will push you around anybody of water just at a slower speed. They are what they are, a lawnmower engine sitting on top of a lower unit. That said, they have their place. Briggs does NOT recommend trolling with them, how ever nothing in the manuals says you can't. My friend in Boisser City, has one that he likes cause no mixing of the gas and oil and he can putt around on Cross Lake aka Crappie Heaven all day. But to me it's just way to noisy. It did push my 1432 really good though