How have I missed this thread for so long?
There is no such thing as 'waterproof' in electric work. I don't care what the material is,or how the connections are made.
This is because the wires themselves are not waterproof ... moisture can wick through the strands. This is because the pipes are all 'open' where they enter boxes. Moisture will condense, then accumulate in the low spots. As a consequence, even in the desert, buried pipe is considered a 'wet' location.
The temporary plastic cover that comes with floor boxes has not the strength, or sunlight resistance, or gasket needed to make it an acceptable permanent cover.
So, you need to take a different approach. Rather than trap water in, you need to make sure it drains. Here's how it's done:
A "handhole" is a fiberglass tube, with a cover for the top only. Approximately the size of a 5 gallon bucket, it has a rather substantial flange for anchoring it in place, in the ground.
Typically, you will dig a little deeper than the handhole, and put a layer of gravel down. Not only will this let water drain, it will help keep down the amount of roots and bugs in the thing.
Your pipes do not connect to the handhole; they will either come in the side, or (better) come up into the bottom. You will leave plenty of wire to work with; 'loose wire' 18" long isn't unusual.
For connections, I use ordinary wire nuts, then dip them in 'scotch-kote.' I take care to place them in the handhole so that they drain.
The handholes sold for electrical use are somewhat sturdier than the ones used for lawn sprinkler valves. The covers are available with a variety of markings. While the $50 price may seem a lot, the installed product will survive being driven over by the gardener, and will not shift in the ground.
Attempts to use other products - floor boxes, bell boxes, etc - always result in any metal corroding, the box (no matter how gasketed) filling with mud, roots, etc, ... and the pipe breaking where it connects to the box.
Another problem with using these smaller boxes is that your pipes are never deep enough. Code issues aside, shallow pipes get broke - usually when the gardener looks for a leak in the sprinkler system. You want your pipes at least a shovel-blade deep in the ground.
Finally, 1/2 PVC was created by the devil, to make your pulls hell. You can forget ever being able to re-pull wires. ALWAYS go up a size or two.
