Lets not confuse the issue, nor compare decking with a glasser. This is a specific request on an older 18' tin can, which I also happen to own one and restored it.
Our floors are cut and our boats are designed to allow for drainage from either rain or splash.
The forward 2 pieces of plywood, basically midship and fore are scalloped on the edges to compensate for the lateral ribs. This promotes drainage as well as airflow.
The aft of many of these boats have a wall, either sliding door or curtain that separates the cockpit from the area underneath the splashwell. This area drains to the bilge.
The poster is talking about an area that starts under the consoles to the end of the finished cockpit. The L trim original on the walls were meant to promote water flow/drainage to the rear while giving a nice fit finish also.
As I said, the plywood is meant to slide under this L which includes the floor covering, instead he ran his finish floor over the L flange, and is seeking a trim solution.
In my restoration which was carpeted, I did the following:
Unscrewed the SS screws that held down the sidewalls and removed them, most Starcraft guys do the same. Laid the decking and used a combination of rivets and SS screws to attach, all rivets on the outer edges into the ribs.
Every seam from multiple deck pieces that ran laterally where the strapping is was sealed with waterproof sealant. After that set, each seam was taped with NASHUA waterproof butyl tape. This stuff adheres and is waterproof. The goal is to attempt to replicate a one-piece deck. That is the point of these tinnies having strapping to join pieces. I even went further before decking and braced the stringers at each lateral deck joint with aluminum for a nailer.
Laid the carpet and then reattached the sidewalls which further pinned in the floor covering.
These boats were designed in this fashion, just go to the Starcraft restorers section and you will see most of them done the same way.
