I also believe that concrete structures absorb heat and retain it, a bit like those old storage heaters. This heat is absorbed by the sun in the day and leaks into the water at night. A very good canal fanatic told me this once and he new a lot about canal fishing.
This could be one reason why bridges are good hot spots especially in the winter. One bridge swim I fish in the winter holds some good chub amongst other fish. Its pretty good all year round but the chub disappear in the summer.
The same could apply to canals in urban areas where the banks are concrete lined and especially any concrete bridge areas. Added to the boat heat, I imagine fish will seek this out.
I look after a big fish pond for a mate, and although inactive now, I observe how the fish always seek out the warmest spot, this time of year, even if its only a degree..
By the same token fish in the canal may hang close in alongside any concrete structures.
Just an added thought, i notice slow worms in my garden always seek refuge under concrete slabs, piles of bricks etc. they are cold blooded like fish.