the specific heat of water is one of the highest of all substances.why does this make water a useful liquid to use in hot water bottles?

and

could you use a lump of nickel to warm your bed instead of a hot water bottle?which would be better? why?

I will be happy to crititque your thinking. Post it. The answer is to think on exactly what thermal capacity means.

The higher the specific heat is, the more heat can be given off before the substance cools back down. The specific heat of nickel is only 1/9 that of water. Nickel would release only 1/9 as much heat as water while cooling the sama amount, assuming the same mass is used.

so nickel would only release 1/9 amount of heat compare those to the water.and when the nickel is cooling down, it would absorb the heat back in.so that's not a good idea to put nickel to warm your bed right!
don't nickel cool down faster than water?
thanks for the help!

Correct. Less heat capacity of metals, means it cools rapidly.