My OH laughed when I used to bathe the boys, as I would get decked out in rubber gloves, a rain coat, apron, towels ready! Two of mine are nightmares to bathe, as they scrabble about and jump out of the washing up bowl I had for them every 2 seconds.
Then I had a brain wave, maybe a shower (obviously on the gentle spray) and the WHOLE bathtub for one guinea! I plonk the guinea in the bathtub with a dish cloth under him and give him a shower - the little monster sits there happily as the water sprays on him and I direct the shower head as needed! When washing his belly, he likes to have a little drink of water... Maybe this is something you could try if you have a bathtub and shower?
Maybe some guineas hate baths and freak out?