Underground splice kit.
Underground conduit is NEVER dry. Consensation will make them wet even if completely sealed.

The boxes WILL fill with water. They are also supposed to be accessable without excavating any material (except gravel, sand or landscaping mulch/similar - and only then if there is a permanant marker to locate the box)

Underground splice kits are a must here but the reality is the box probably isn't big enough to contain them. A material like Scotchkote might do a reasonable job of sealing the splice but they will need several coats and you won't know for sure it worked or not till you have a failure.

The best way to do an installation like this is to use pullboxes similar to the boxes used for underground sprinkler valves. The conduit enters the open bottom of the box through 90s and the splice is made in the empty space between the box lid and the fill in the bottom of the box using underground splice kits. The fill is sand or gravel and it will drain away standing water so the splices don't sit in water all the time.

I'd really suggest you do something different even if it's install larger boxes than you have so UG splices can be used. If you have aluminum wire and it gets wet you're gonna have paste where the wire was real soon. If it's copper you might wind up with some exhorbitant electric bills and not know why as well as voltage issues on the load end.

please don't say it is 6/3 NM Romex cable. If it is, it will be shot in a year

It just regular 6/3, non underground cable from lowes.. heavy vinyl type outer insulation. It is in 1.5 conduit...

6/3 nm in a conduit full of water is dangerous and won't last long.

It just regular 6/3, non underground cable from lowes.. heavy vinyl type outer insulation. It is in 1.5 conduit...

Might as well pull it out now and start over. Use individual #6 THWN conductors this time with no splices.