Fiberglass Tower Build for Boat

I am going to be building a tower enclosure for a buddy of mine that will use 4 x 6.5" Coax apeakers. He now wants to add an 8" sub in the middle that will look like this:

Once he purchases the sub I will build the sub enclosure first, according to the volume specs of the sub, then incorporate it into the rest of the build. It will be powered by an amp that is run off a seperate battery and I plan on running the 6.5"s off of the deck.

My question is should the sub be ported or sealed? I have built many fiberlgass enclosures in the past, but all for car-audio applications, never in an "open cabin" environment. He doesnt listen to rap or hip-hip, mostly old school rock so he isnt looking to host a cove party, just rock some oldies with a good hit. What type of enclosure will I need to get the most response from a single 8" sub?  If I port it, it will likely be a single cylinder that fires down using 2-3 in pvc.

Any suggestions? Thanks in advance!