With 5 on board and doing more water sports and runnin' - occasionally fishing (if I follow your order right), look for something about 18'. And I would lean toward a buncha' power behind it. That'll cost you some in fuel economy, but not enough to be a killer. Don't fret about who manufactured the boat, look at condition and at the power behind it.
THIS topic is pure gold when shopping for a boat. You may also want to rent a boat or two (if you have that available) to try 'em out and see what you like and don't like about 'em.
The boats I'm gonna' link to here are not suggestions, just examples: a boat like
THIS would accommodate 5 people and would be a nice type of boat to fish from, but wouldn't do well for comfort or skiing/tubing. (You can kinda' tube behind anything.) Dunno' if the shirtless model comes with the boat or not - buy at your own risk
Something like
THIS one, would still accommodate 5, be a little more comfortable, better at fishing than other stuff but would pull a skier or tube okay.
A boat like
THIS one or
THIS one is the most common thing people are getting. It's more comfortable than the others and should have good power for pulling a skier or tube with 5 people on board. It's not gonna' be designed to fish off of, so it's a little inconvenient in some senses ... but then again, the fish really don't care what ya' haul 'em up into, right? We fish off my bow rider rather regularly. It's just not the boat's forte.
My boat is a 17' and we usually go out with 5 people (two adults, three teenagers) and 2 dogs. We're not cramped in, but we don't have a ton of space left over either. At the same time, I trailer my boat and the 17' will fit right in the garage with no trouble so it makes a very convenient boat to have around - so we use it a lot more.
All kinds of stuff to consider as you start lookin'.