|"Somewhere I read to soak the cartridge in water but I'm not too sure of the advice"|
Whoever told you that gave you bad advice.
If you suspect a blockage in the ink delivery system which can't be cleared with the printer software's nozzle-cleaning utility, you remove the printhead and soak that in warm, soapy water (with all the cartridges removed). Then rinse it and leave it to dry naturally before re-fitting it. Never soak the cartridges!
Blockages which can't be cleared with the printer software's nozzle-cleaning utility is usually a symptom of the printer not being used frequently enough, thereby allowing the residual ink within the printhead to dry out and turn into a semi-solid state.
Some brands of printer are worse than others in that respect in my experience. Epson printers are particularly prone to it, whereas Canon printers are much less prone to it. Two different ink formulas probably accounts for that.