When I saw the thread title, I thought: "I bet she's pregnant..." It's fairly common for your milk supply to dip and for the remaining milk to get extra salty. DD1 really upped her intake of fluids when I was pregnant with DD2, despite also increasing her nursing as well.
I see that you are worried about her weaning because of this. Some children do self-wean in pregnancy due to the change in supply/change in taste. Others do not. Some moms have found it helpful to go back to the "early & often" strategy of offering nursing to your child. Continue to offer nursing prior to other beverages. She make give up day-time nursing and only nurse to sleep/at night. She may give up all nursing. She may decide it's okay to just check in with you by nursing really quickly and then getting her thirst quenched by water. Just keep offering!
Adventures in Tandem Nursing was a nice book to read early in my pregnancy, just so I could read all the different Moms' experiences. I really didn't want my DD to wean (she didn't, FWIW), but I wanted to prepare myself for the possibility that she might.
Nessa, DD1 (5) DD2 (3) & expecting again in late February/early March!