A few opinions on changes I'd like to see regarding the fields, based on logics and anime animations:
Water surface:
Dive should not have changed it to underwater. User of dive is the one diving, not his opponent, nor have he dragged his opponent down with him (assuming opponent doesn't chase after him, which is usual). Dig functions the same
Dive last as long as the user does not choose it again, instead of 1 turn. Only the user takes all the effect of underwater field (including increased water pulse bp used)
-Surf/muddy water should be the ones which change it to underwater field. Surf=They summon waves of water to drown the opponent in anime
Seismic toss change the field to underwater field, and deal 1.5x the damage
Sleeping/frozen pokemon takes the effects of underwater field
Smackdown sends the opponent underwater. Basically the opposite side of dive
Ice beam changed the field to icy field, as seen in anime
Electric moves shall not miss. Water conducts electricity, get it?
Accuracy against underwater pokemon from the surface is reduced to 75% (except electric moves). Dive doesn't result in the semi-invulnerable turn like always. It instead put the user at underwater field's effect only
Contact moves used against underwater pokemon changes field to underwaterfield
Underwater:
Dive/fly/magnet rise/bounce brings back only the user to the surface and enjoys all surface field effects. Effect stays as long as they are not used again.
Skydrop brings both user and opponent back to surface, ie change the field to water surface. Damage of skydrop factor in water type damage. (you throw a water type onto water surface, don't expect high damage. Opposite for fire)
Electric moves will not miss
Pokemon with levitate are unaffected by all effects of underwater field
Surf/muddywater deals half the damage
Pokemon not water type suffers 1/8 (weak to water) or 1/16 (neutral) damage each turn underwater. They can't breath, so the drop in vitality. Sleeping/Frozen pokemon takes double the original damage (unable to control themselves)
Freezing chances of moves becomes 100% (only move with initial freezing chances, even if it's 1%)
All rock type moves (except headsmash and smackdown) and moves affected by bulletproof ability deal additional water damage
Smackdown deals additional ground type damage
All physical moves used by non water/dragon type or non swimmer (including those mentioned in previous points) suffers 75% lost in power. Water drags
Accuracy against surface pokemon from underwater is increased to 1.5x
Contact moves used against surface pokemon changes field to underwaterfield
Solarbeam takes longer to charge
Frozen duration is halved. Water thaws too
Electric moves hits all opponent and ally in double battle
I'll add on if I have time to think of others.
I really think that custom field effects are nice ideas, but a lot of work have to be put into thoughts and brainstorming if you really wanna make it effective.
It's like Ame said earlier about creativity with manipulating fields. I would have used seismic toss on a surface Blissey to go underwater and heal up those damages stalling, or would have magnet rise my slowed down underwater Electrode to spam electric moves .