The planned landing areas for Apollo command modules were pre-programmed beforehand and happened in the South Pacific.
Often the South Pacific, but sometimes the North Pacific or the Atlantic. Both Pacific and Atlantic recovery zones were established for each mission, with multiple ships allocated to each.
Was there any remote possibility of a splashdown happening in the USSR controlled waters and what procedures were in place for that event?
There's relatively little USSR-controlled water per se on the planet's surface, so that would be an extremely unlikely occurrence. Slightly more likely would be landfall on USSR-controlled soil (simply because there's much more of that), but as Uwe points out, the ground track of the usual Apollo parking orbit never reached the high latitudes of the USSR; Apollo 11 for instance inserted into a 32.5º inclination orbit; the southernmost point of the Soviet Union was Kushka (now Serhetabat) in Turkmenistan at 35º north.
In general, landing somewhere unexpected was very unlikely. An abort prior to getting into orbit would leave the command module somewhere in the Atlantic. Once in orbit, any abort would be performed to land near one of the established recovery zones if at all possible. If the abort wasn't now-now-now urgent, the worst case scenario would be maybe 30 minutes from the abort decision (say, over north Africa on a northerly leg) to the point at which the CM could reenter over a big ocean.
But let's say the worst happens, and the CM has to do a prompt emergency reentry without any choice about where to land. I don't know exactly what contingency plans NASA and the rest of the US government had for this situation, but I would expect that American astronauts crash-landing in USSR territory would not be mistreated even during those Cold War years. The Apollo missions were well-publicized, clearly scientific rather than military, and "the whole world was watching". The astronauts probably wouldn't be invited to tour any sensitive facilities nearby, obviously, and the command module might get impounded, inspected and dismantled before eventually being returned, but the crew would almost certainly get home safely.