This one needs a bit of explanation. The kanji literally read "water hermit", BUT are actually the Japanese word for "daffodil" [suisen] spelled with the kanji in reverse order. I find it odd that Sensui would be named after a flower like the ferry girls (Ayame, Botan, Hinageshi), but spelled backwards. Maybe it has to do with his position in Reikai (the flower) and his denial thereof (it being backwards)?
This is a fairly commonly given name in Japan for both sexes. However, this kanji spelling that Togashi employs here is more often given to females [males usually use another kanji].
Its meaning, quite appropriate for the character, is "to endure" or "to be patient". Sensui has endured quite a bit in the series, from the contradictions posed to him by the fateful raid on the Black Black Club to his insanity. Also, interestingly enough, this kanji [in another reading] is the "nin" in "ninja" and connotes stealth. (Resshuu-ken, anyone?)
Also, "Shinobu" is another plant-related name. It can refer to a species of fern in Japanese (thank you Denshi Jisho!)
