What is the best way about re doing the carpet in my boat. It has carpet in in already. Putting the new in im not sure how much to buy or the kind of flue or adhesive to put under the new carpet. 

Depending on what boat you have there are some places that sell pre-cut carpet for your model. I would also use a marine carpet adhesive which can be found at most marine suppliers. 

I'd agree with WIGuide's post, which would definitely be a help if you wanted to do it yourself or make it easier for the shop doing the work.  The forums and You tube has videos which may be helpful if you'd like to do the work yourself.  Be aware marine carpet come in various "weights" depending on manufacturer, which can come into play regarding the wear life of your carpet.  I just had my boat's carpet replaced and the shop was going to use 18oz carpet when the boats manufacturer uses 24oz carpet.  And if you're gonna buy the carpet yourself, as the shop explained to me, the carpet can be sold by the "running" yard or "square" yard.  Good luck with your project

here is where I got my carpet and adhesive.

https://www.bassboatseats.com/collections/bass-boat-carpet

redid my boat and put in all new deck boards. pretty easy work just time consuming. we did mine in 3 or 4 nights. youtube will be your best friend along with google. we used indoor outdoor carpet glue and marine carpet.

the hardest part is cutting the corners around lids. also get a good sharp carpet knife. when you cut your piece out always use orientation arrows on the back side to distinguish which way the piece will lay. if you turn the carpet wrong it will stand out.

i think altogether we had about 400-500 in mine with glue. carpet. and new deck boards.