Is thermally conductive epoxy needed if long settling time is acceptable?
What you need for accuracy is a large difference between the thermal conductivity to the water and to the surrounding air. The hose will be a pretty good insulator so I don't think the epoxy will matter that much. Accuracy will be slightly affected by self-heating but probably not to an important degree (so to speak). 

For fast response you need low thermal mass and low absolute thermal conductivity 
to the water. 

Metal-loaded epoxy like JB weld is available in consumer-friendly packaging and prices, but not sure about its insulating properties. 

If I might suggest- make the back of the PCB flat and pour an insulated plane, screw it to an aluminum block with a semicircle cut out of it and a flat top, attach the block to the clamp with screws. No epoxy of any kind. Then surround the assembly with foam insulation.


The best solution, of course, is to cut the hose and put a small-diameter sensor (maybe not that one) in direct contact with the water. If the L/R ratio is not high enough (10:1 is good), then bend it so it goes in line with the water.