I'm guessing this is not a hobby request (based on the photo of the manhole cover). I've looked at pressure sensors for level detection many times for automotive applications and can offer the following:
o-rings don't seal. For your application you need either a hermetic seal or fully potted assembly
open tubes will fill / drain based on condensation temperature changes etc.
Freezing is often an issue but I'll guess at you depth you don't have to worry about it.
Have you considered a pressure switch? It wouldn't necessarily solve the seal problem but might work for you. Still thinking.... the pressure switch might be hard to self test. At least on the pressure you can see small changes, suggesting the transducer is functioning.
you might find this link useful: submersible transducer
If you are looking for a lower cost solution, look for "absolute" pressure sensors as opposed to "gauge" pressure sensors.
If you want to go the potted route, you might look at this: [link text](absolute xducer). If the cabling was completely potted in a suitable material, this could work for you.