Hi th, yeah it probably is the clutch in some way, this sounds just like the clutch is not disengaging completely. Try pushing the clutch pedal down and SLOWLY trying to engage reverse gear, if it makes chrunchy noises then the clutch is still slightly engaged. Usually this is caused by wear of some kind but in your case it might be rust on the shaft the clutch disk slides on due to the water entry, I think that most likely you can't reach this to lubricate it and if it continues you might have to consider having the cluch out for remedial work.