Don't bow to my wisdom.
I have found that, across my gaming time. Death gems, blood slaves, air gems, etc. They all seem somewhat similiar with the exception of Fire Gems and Water Gems in terms of raw quality of potential uses (Summons/Items/etc).
Death certainly has no weakness in the variety it can use it's gems, but I wouldn't consider it the 'best'. I have found nature to be the same way, if only for the fact that it has a good, semi-unobtrusive global (Mother Oak) a few game-winner globals (Gift of Health with Tartarians, Haunted Forest) and a plethora of good items for gems (even if it's only Hydra Armor and Rings of Regeneration for magic items).
I have certainly had a great amount of successes with nature gems and in fact I have found it as 'needed' a type as Death/Blood for my strategies.
Clams: I don't particularly think that 2W is too cheap for Clams, the feeling or desire to make an Astral income from Water really comes from the lack of good gem uses for Water as much as Astral is a great and versatile gem type that can be fitted to a need at any particular time.
There is a change to them in my Magic Item mod, if only to make potential 'hoarding' investment less attractive while keeping a 'small Astral gem income with Water Gems' availiable. It's not a complete fix, but I think that particular debate has gone round and round and there is no good answer. Some people will always horde and will always try to set their strategies around it, seeing it as a potential payoff while other people will not let their weakness be so easily exploited in the early game.
I can certainly say it's boring hording (making clams is a sucky version of micromanagement some *feel* compeled to do in order to feel competitive) and it's boring fighting a hording war (Way too magical for my taste) that is less about how your fighting and more about how often can you go back and forth with your army-destroying-SC-killing-hyperempowered-Pretender-God-Too-many-path lameness that tends to degenerate into a stalemate. Stalemates tend to suck the fun out of Dominions 2 I have found. Even though you agree end the game, there is no sense of closure, which tends to irk me especially if I have devoted alot of time to the game (as Dominions games tend to).
Either way I'm rambling. I have revised some changes, I will release the final Pretender Mod (some minor typo corrections, slight change to a few Pretenders, nothing crazy), v1 Summons Mod (only adjusts summons statistics, not spell costs) v1 Spell Mod (only affects costs and spell effects), v1 Nations Mod (welcome to an attempt in variability and to make nations have gold be a factor in the late game outside of building mages and castles a pretty hefty task considering capitol only limitations) and the v1 Magic Items mod by the end of next week, if I'm lucky.