what is considered a seat

[archer379]

My wife and i were talking this morning about how many people we can have in are boat (17.5 crestliner fishhawk) the tag says up to 6 people or x number of pounds. however there are only 3 pedestal seats. i have read somewhere that you must have a seat for everperson in the boat would the rear casting deck count as a seat in my opinon it wouldn't be any differant then a seat in a tin boat. we are planing to do a bit of tubing and want to take my daughters friend with us do i need to install another seat.