There will be condesate as the cylinder is heating up.Natural gas 10% water vapour and LP gas 25% water vapour.If the pilot is not holding or only holds for awhile it might need replacing
 also pilot might have dust on it making flame yellow
 - uni-control could be failing- how old is cylinder

  Get a Gas Fitter to look at it as you need it looked at safely

Sounds like the Thermocouple needs replacing. Rheem still sell them as very common issue.

But then again most plumbers I talk too immediatly suggest the thermocouple for every gas problem hahaha

Very true dragon 

just replace the whole HWC 

What i say may not be the issue, but on top of the cylinder there should be a draft diverter, that needs to remain clear at all times, so many people put their clothes around it to dry them off, the effect of that is the products of combustion dont leave the flue to atmosphere and in some cases where downdraft occurs can snuff out the pilot due to lack of air for combustion;
The thermocouple is the usual fault and they are cheap as chips to buy and simple to fit, but in doing so one might aswell as part of a routine clean/check the pilot injector, after assembling and checking for leaks on any pipes undone,ie. the burner and the pilot tubes light it up and adjust the pilot length if it is bigger than a bic lighter flame its more than likely impinging itself on the thermocouple, again upsetting things, all very simple stuff, your main burner should come on and stay on till the water reaches temperature then the burner turns off leaving only the pilot going, just to clarify the burner doesnt go all day its whats refered to as snap-acting not modulating like more complex appliances.