Just wondering, when these boats are made they make a upper and lower part, then join them together. How do they bond them? Anyone been in Manufacturing?

When you say "these boats".......

High end hulls, your Eliminators, Bahners, Millers, Rogers, Sanger etc.... The outer perimeters are the same size and shape so the mating surface is a "butt joint" and more desirable. These are glassed together fairly precisely and can either be capped (finished without molding) or a rub rail (gunwale molding) will be added.

On the entry level hulls (forgive me if I offend) Bayliner, Rinker, Marlin, Galaxie etc... The outer hull perimeters are of a different size and the deck overlaps the hull, these are typically just riveted together and the rub rail is added.

Oh, and what Ron says too... 

When you say "these boats".......

High end hulls, your Eliminators, Bahners, Millers, Rogers, Sanger etc.... The outer perimeters are the same size and shape so the mating surface is a "butt joint" and more desirable. These are glassed together fairly precisely and can either be capped (finished without molding) or a rub rail (gunwale molding) will be added.

On the entry level hulls (forgive me if I offend) Bayliner, Rinker, Marlin, Galaxie etc... The outer hull perimeters are of a different size and the deck overlaps the hull, these are typically just riveted together and the rub rail is added.

Oh, and what Ron says too... 

When I worked for Sea Ray here in Phoenix the deck was slightly larger than the hull, calking was then placed on top outside edge of hull and then deck was lowered onto hull, holes drilled every 6 to 8 inches apart and then screwed together with stainless hardware then a rub rail (gunwale molding) installed.  No gap under bottom of rub rail could be larger than a dime.  This same process was used from lower end 17ft bow riders to 30ft cabin cruisers we made.  The we put every boat in a gian swimming pool and checked everything for leaks and ran boats in pool to check for leaks and operating systems. had a car wash type set up the pulled over front of cabin cruisers and had to be ran a min. of 1 hour to check deck, windows, etc. for leaks. just some FYI. Sea Ray ended up closing the Phoenix (original) plant to mover operations to Tennesee and Florida.