+1 on this.. I do this for a living all day every day, Fiber is the way to go.
its not that expensive. you will need either data switches at each end with SFP modules in them or you can use something like Transition Networks media converters at each end to convert Multimode fiber to ethernet copper.
Here are some caveats. ALL underground conduits are full of water from condensate, suck or blow it out if you like, it will fill again, matter of time.
the cat5e duct cable is one way to do it. but it invites lightning. ALL underground copper cables are prone to lightning. proper arrestors and grounding techniques must be used, from a personal protection standpoint and protection of the equipment. If you dont, use a duct cable, it will fail, matter of time for the water to ingress into the "porous" PVC cable.. with the cost of the lightning protection and the cat5e duct cable, you are pretty close to the fiber price. the fiber itself is cheap, but there is the splicing ends on it and then the media converters or SFP modules on the data switches.
Fiber has no lightning problems, I would still use a duct type fiber since it will be laying in water for a long long time. If you need help with part numbers or ideas PM me. Just my thoughts.
James K0UA