Risen Riptide’s ability overwrites any previous effects that set its power and/or toughness to specific values. Other effects that set these characteristics to specific values that start to apply after the ability resolves will overwrite that part of the effect.

Effects that modify Risen Riptide’s power or toughness without setting it will apply to its new base power and toughness no matter when they started to take effect. The same is true for counters that change the creature’s power or toughness.