For contrived setups like this, I like to turn to my damage calculator, which handles peculiarities that none of the popular calcs in common use do. In particular, this scenario is very similar to one that I fed to former world champion Wolfe Glick a while back for comic effect, and some of the same oddities in the damage formula arise here as well, albeit less visible.
Getting these swaps in place (such that, among other things, Kyogre can be at +6 with a spliced ability and still be able to choice-lock into a damaging move against an opponent who's at -6) requires a double battle, and Water Spout is a spread move. Even so, it's possible for Water Spout to avoid getting the spread penalty, but only if Slugma's teammate has fainted, leaving only a single target for the move to hit.
With the spread penalty applied, the list of possible rolls (in order of increasing damage, which in this case is NOT the same as ordering by increasing random number during the random step) is: 548, 3562, 9516, 15448, 18484, 21382, 24416, 30350, 33384, 36282, 39318, 45250, 48286, 51182, 54218, 60150.
Without the spread penalty (so with Slugma's partner out of the way in advance), you get these figures instead: 826, 4688, 6758, 12690, 18624, 20694, 24556, 26626, 32558, 38492, 44424, 46494, 50356, 52426, 58360, 64292.
The choice of Slugma is also an odd one. Prior to evolution, Slugma is just a mono-fire type, as opposed to Numel which has both types throughout its evolution line, and thus the double weakness to water. If you really meant to use Numel instead...
With spread penalty: 1096, 1232, 7028, 7164, 13100, 19032, 24964, 30896, 31036, 36828, 36968, 42764, 42900, 48832, 54764, 60700
Without spread penalty: 1652, 9376, 11448, 13516, 23312, 25380, 27452, 35176, 37248, 39316, 41388, 49112, 51184, 53252, 63048, 65116
Compare with the absolute maximum damage cap of 65535, which you can never actually exceed with any move under any circumstances. Any damage calculator that tells you otherwise isn't taking these obscure factors into account.
