1. Ice melts leaving water at the bottom of whatever fuel it is in, making it difficult for it to evaporate.
2. I don't believe all of the water is going to get out of the engines' fuel systems. They are not open to the air so again, this makes evaporation difficult. The pipework in the engines was pressure and vacuum tested, so no leaks for the "miracle water" to escape from.
3. Jet fuel is hygroscopic. If you have free water in jet fuel, you also have dissolved water, which takes a while to get rid of and is easily detected down to very low concentrations.
There can be no escape for the saboteur water droplets. To believe it was water that brought down BA38 you have to believe that the AAIB, being suspicious of the quality of the fuel and aware of the forensic importance of the contents of the fuel systems, botched the examination of the engines very badly indeed.