In regards to the diurnal water triplicity ruler, i use Venus. In regards to the Question of the bearing this has on Kirk's Grand Water Trine, i would say to treat it in a manner consistent with any other use of it. The only relation i see (based on your input and not looking at the chart) is Venus despositing Mars by exaltation. the trine indicates the relation these planets/signs/houses have to one another, how these areas of life interact with each other. using Venus as a triplicity ruler for prediciton would bring the Mars (through Pisces) into the arena of expression, but i don't think Venus is going to "activate" this GT.
on another note, relating to Mars in Cancer. i have a friend with a partner who has Mars in Cancer/first house! while explosions are almost non-existant, he is constantly at a "low-boil" and takes serious emotional offense to most things (cannot let go of hurt feelings, very scorpion).

EstebonGone,

Thanks for your thoughts. I have been catching myself making my choice of Venus or Mars based on my own chart. All these planets in the water grand trine have tempted me to decide by judging which feels right based on my own experience?probably not the best way to develop a sound and flexible understanding. If I can detach myself from the water element I just might finally come to a decision. For now I will just let the two battle it out for the position.

Kirk