How do you know which R himes can be obtained in gem gatcha? I have spent around 40k+ gems and always seem to get the same r unlocking weapons. I really need more water himes to complete my mono water party and i heard there is a r water hime that can heal ( my main and only healer is nike)

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Currently im running a water team with nike, gabriel, rusalka, cybele (i know she is wind but is a good debuffer) cassiopeia, with a 9 sr water grid around lvl 8 skill in average and 23k atk. Im considering going for andromeda (already have arthur) but siegfried seems good too. Which would be better? Nike is my only healer hime and my chances of getting a ssr hime are really slim at best. Thx in advance