After a month on lowest dose of spiro I had a bout of severe dehydration too - spent a whole day describing what I was feeling to nurses, who were for some reason baffled - I didn't use the magic word "thirsty" so they were at a loss; like the OP I just didn't feel especially in need of guzzling down fluids. Being in alright shape but not a fanatical fitness person or involved in much of anything in the way of manual labor besides playing the accordion I just don't work up much of a sweat, so didn't have familiarity with what it feels like to be dehydrated - and I had no idea you could be dehydrated without feeling thirsty.
My circulatory system felt like it had turned to mush, especially in my lower extremities; the only remedy was to lay down flat on my back. I toughed out a whole 8 hour shift at work like this, managing to get up when needed; after trying in vain to get an answer from all those nurses I staggered up the road to an urgent care clinic, I had to lean back in the driver's seat as far as possible to keep from passing out, and the really condescending doc identified my problem, which I'd already managed to solve by rushing over to Subway, back to the hospital ASAP ("If you're not back in 15 minutes you'll lose your spot"), wolfing down food/drink. Voila, feeling OK now. Those salty chips tasted so good.
Now, I'd been drinking my usual quota of fluids all this time, and felt fine, far as I could tell, and adding the extra salt everyone says is critical. Things did seem to settle down after this episode. I'd already paid a visit to the ER for another problem, if I'd wimped out and gone back I'd have had to wholly abandon HRT and transition, the cost would have just swamped me - I've only recently managed to pay off the cost of the first visit, I'm a low income earner. An HRT FAQ would be invaluable, there are so many little details like this that could be easily avoided, and the documentation that's out there is so spotty.
Spiro has never made me urinate constantly like it does to some, either. YMMV as ever. Upon waking I drink a whole pint of water, with some salt thrown in, and pee a bit - my prostate has shrank so I'm never woken up in the middle of the night having to go. Through the day I drink another pint of water every other hour, I treat it like another med.
At first I had fairly constant leg cramping, but almost never now - I don't think I excrete much salt, unlike some.