You can only lower ground up to 8m from its initial position, if the terrain is still too high then you’ll have to go around it. You might have more success canalling from the swamps directly to open ocean, as the swamp shouldn’t have any areas that prevent digging into water

Thank you Nascarman very much !!!

What I did was to explore the entire swamp and mark the crypts before doing anything else. Then I sailed my long ship as close to all the crypts as I could get it (some center point between the crypts that I found). Drop a work bench and a portal so I can repair the boat and then drop a few more workbenches around the area so that things don't spawn in that area when I'm walking back to the ship.

Build a work bench and a chest or 2 outside the crypt plus another portal and go in and explore and mine. When your pick breaks or you are at capacity go out side and put your scrap in the chests and portal everything else back home. Get dry and get your rested bonus back and take the portal back to the crypt. Once the crypt is clear carry the scrap to the ship and put it in the cargo hold. On your last trip dismantle the portal by the crypt and mark the crypt as cleared on your map.

Repeat until your cargo hold is full of scarp, you have emptied all the crypts in that swamp, or you are tired of grinding scrap, dismantle the portal (or leave the portal if you are going to return later and get more scrap) and sail the scrap (and portal parts if you dismantled it) back to your base.

After you unload the ship at your base, if you left the portal for later, you can chop the ship to bits at your base to recover the materials and then portal back to that swamp with the parts and rebuild the boat.

thankfully black marble can be teleported.... otherwise i would have been screwed by this point. already spend over 5000 marble into my new main base. found the perfect spot for it. a small patch of plains near a channel on a huge long island which has all biomes except mistlands (which are giant on the other side of the ocean biome in front of my base. its just perfect. but the landscaping has been going for days and there is no end in sight.

but takeing trips into the mistlands to find more marble has been a really fun experience. gotta be always on your toes in case something comes out of the mist while youre mineing blindly.

learn your topography, know it like your hand before you get 1/3 in the project.

while i don't use mods and prefer to stick with how the game was intended i know that there is a mod on nexus mods that removes the max dig limit, just in case you've spent a massive amount of time digging the channel only to find that 1 bit screws all your hard work.