One thing to remember about any buried conduit: it will accumulate moisture regardless of how tightly you seal it. This is because air temperature differentials will inevitably cause condensation. It may be slight, and will vary depending upon your location and how deeply the conduit is buried, but it will happen. There is no cheap way to absolutely guarantee that no moisture will get to the conduit.
I like the suggestion of using PVC pipe conduit. I've done this for years and it works very well. Just make sure you construct the conduit very carefully, and get good seals on all the fittings. An extra layer of solvent cement on the exterior after the joints dry is always a good idea.
The icy-pic cabling is intended for exposed exterior use. Exposed use is not a good idea regardless of location, but is on rare occasions the most economical choice.
If you are wrapping around other cabling, be aware of the current it is carrying. Ethernet does not like stray voltages, especially those running in the same direction. If you can (and this is highly unlikely in a buried conduit) rotate the CAT5 in a direction opposite the pairs of any other twisted/stranded cabling.
I use solid Cat5e non-plenum 4-pair w/ standard jacket in PVC conduit, adhere to Ethernet specification distance parameters, use good quality switches (never hubs), and pay special attention to how well I terminate each end of such a cable run. I have never, ever had a problem.
Mark