some questions (exhausted video memory? + how to underwater building?)

hey fellow world creators,

im trying to build a big open world game thing. all on youtube videos:P getting some nice islands now and trying something biger, and biger.
now im at the point i max out the space and i like 2 islands, a brige is easy and now i want a tunnel. how do i keep the water out? i just downloaded blender try to make a nice tunnel for a spline.
bud need something to keep my screen from turning waterbleu.

and is there a way to not load the complete world all the time only sections? when i put texture on my spline on a big map like a road all arround i get the error video memory has been exhausted? unreal keeps the full worlds loaded all the time by the looks of it?
now it stoped uploading textures on the roads. functions great while flying arround in the map tho. (go a acer predator helios 300 7nth gen intel i7 520hdd and 64 ram) to slow? i bet it is bud still?

i love unreal 5. the more i do the easyer it gets hahahha only its hard to get a lot of car’s only basic not real cars. some one know a good spot for real cars? like a skyline r34 or lamborgini? and not pay full game+ extra game money?

best regards

oke i found that it may be better to open 4 new landscapes and make it big than open one big one. i can hide 3 off them now. big downside at the moment is spline cant continue and sculpting only works on the selected landscape. hope it works for me!!! slow ■■■ comp/ or freaking to big and to fast program:P
try some things. still need an idea for the water?
can i make a air cube? is this strange to tink? figuring blender out. can make some parts bud getting an image on it still hurts my brain

thanks in advance

whahha just use the eye on landscape in the right menu