ABS plastic is basically the same thing that LEGO’s are made out of. It can fade over time, and certain colors are more susceptible to that then others, but they also do make certain car trim pieces out of it. That being said, fusion deposition layer printing is going to leave lots of tiny hairline spots where algae and whatnot could get in over time and cause separation or finish dulling. You would probably be just fine if whatever you plan to make could then be coated in a gelcoat paint or wrapped in carbon fiber and laminated or veneered, but by itself in it’s own right it probably would not be the best choice for a marine environment without a surface treatment.
As for building things bigger, the best solution would be to make the platform itself bigger. you would need an additional power supply, possibly a second controller, a second heatbed, , longer bearing rods, belts, frame pieces, a second platform mount, and a really long sheet of borosilicate glass. You would have a temperature differential right at the join edge, but the part itself would be long enough that probably wouldn’t matter as much as it would for something smaller. You might be able to get away with a single controller and a really beefy power supply, but you would need someone better versed in electronics than me to tell you exactly what could be done. You would also lose a lot of print speed in the Y axis direction due to the additional weight, so you might have to go with dual Y axis motors, larger belts, dual belts, etc.
If that sounds like too much work, gluing parts, or using fastners of some sort would be your best bet. or redesign the parts so they use the variety of relatively inexpensive carbon fiber, steel or aluminum tubes as bridging members.