Would a flat boat be good for saltwater and fresh water? what size flat boat recomended for a 50-70 hp motor?

The LAST thing yu should have in salt water is a flat bottomed boat. You're just asking to get flipped, drowned, and eaten by critters. Not in 'that order, either. At best, a flat bottomed boat for people should only be used on lakes, in calm weather, and by someone who respects the boat.