This is gonna get a little technical, but I'll try to limit the organic chem jargon. Its actually much easier to understand diagrammatically. If I'm unclear about something, or you're having a hard time with it, let me know
Morphine is complicated. Most drugs have one functional group that ionises (becomes charged depending on the pH of the solution), and for the most part this is the NH3/NH4+ amine group. This group becomes charged when the solution gets more acidic (more H+) because the amine group is a proton acceptor. When only one ionisable group is on a molecule, the molecule carries the charge of that group, so when that group changes charge (neutral <=> positive) so does the molecule. When organic compounds (amphetamines, for example) are neutrally charged, they behave like a hydrocarbon and dissolve (generally) in non-polar (toluene etc.) solvents. When they become charged, they interact more with water (polar, like a magnet - + and - poles).
So, lets say we're talking about speed- its amine group will have a specific pKa (ionisation constant) which, in practical terms, represents the pH value where it is HALF ionised (i.e. half of the molecules are charged, the other half are neutral). As you make the solution more acidic, more of the amine groups will accept a proton to become charged, making a greater proportion of water soluble speed ions. The transformation is functionally complete at +/- 2 pH units (99% converted one way or the other). If you add acid to this simple amine and then dry it out, you have the acid addition salt called 'amphetamine [acid you used, typ. HCl]". If you add a base, you get the free alkaloid (freebase).
Morphine has a second ionisable group, a phenol group (OH/O-), that behaves in the opposite way. Add acid and the group keeps its proton, add enough base and the proton is stripped leaving a charged O- group. This is the morphinate anion, and exists at pH higher than 10. It is water soluble.
To recap, morphine is soluble in water up to pH 9.8 (isoelectric point, 'true alkaloid') as the morphine cation, becomes neutral and insoluble at pH 9.8 (both groups contribute evenly to charge, cancelling each other out), then becomes soluble again above pH 9.8 as the morphinate anion (meconate),
The plant presumably keeps it in anionic form for transport purposes.
To answer the question, calcium morphinate is soluble in PURE water which has a neutral pH. The water doesn't influence the character of the salt because its got nothing in it. If you added pure calcium morphinate to water, the morphinate would change the pH of the water.
If you're adding the morphinate to a buffered solution, that's different cause there are acids/bases that have a greater influence over the solutes.
So the short version is that plain deionised water will easily dissolve the meconate, but if there are other acids and bases in the solution, the charge of the morphine is at their mercy.
I know, long winded and kinda shitty, hopefully that helped a little for someone. I'm better at answering questions about this stuff than explaining it in full, so if you're interested please pick at the bits that I made overly complicated or glossed over.
Paradox
