I think most of the mounting force would be transmitting by the areas of direct block-die contact. Water would squirt out rather than be pressurized. And I think that's why you got good results -- minimal TIM, maximum contact.
The way to find out is to test with a range of mounting force. Compression of the trapped air (less volume, higher thermal conductivity) is one of the ways TIM joints improve with greater force (I don't think water would be as subject to the 'more pressure = thinner TIM' mechanism like high viscosity goops).
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