I think that particular crypt is forever inaccessible due to the nature of world generation. In many hundreds of hours on numerous world seeds, I think I've only come across a completely blocked crypt once. On to the next one!

I figured as much.

Marked it as Pointless on my map .

Thanks for the info.

If it's high enough above the waterline you can try to get into it from the bottom. Dig under the side of the front stairs to the middle section (around the inside of the gate). If it's high enough, you should be able to get under there and still use your hoe. If that's the case, face outward (with your back to the back of the hole you dug) and pile stone just in front of you. That should push you through the bottom of the crypt mesh and into the crypt.

If the crypt is too low (most are), you'll hit the waterline too soon and will end up floating under the crypt, unable to use your tools.