How does Octopus Umbra interact with effects that change the base power and toughness of, and counters on, a creature?
The judge's ruling was correct. The various modifiers to power and toughness are applied in layers, and the 8/8 base P/T are in a different layer than the +1/+1 for each enchantment. With the three enchantments in your scenario (Octopus Umbra plus two others), Tuvasa the Sunlit will be 11/11 overall.
There are many continuous effects that can modify objects. They are applied in layers. Effects that modify power and/or toughness are applied in layer 7, and layer 7 has another 5 sub-layers. They are applied in the following order:

613.3a Layer 7a: Effects from characteristic-defining abilities that define power and/or toughness are applied. See rule 604.3.

Example: &quot;This creature's power is equal to the number of knights you control&quot;.

613.3b Layer 7b: Effects that set power and/or toughness to a specific number or value are applied. Effects that refer to the base power and/or toughness of a creature apply in this layer.

7b is where the 8/8 modifier from Octopus Umbra is applied. It overwrites its original 1/1.

613.3c Layer 7c: Effects that modify power and/or toughness (but don’t set power and/or toughness to a specific number or value) are applied.

7c is where the +1/+1 per enchantment from Tuvasa the Sunlit is applied.

613.3d Layer 7d: Power and/or toughness changes from counters are applied. See rule 122, “Counters.”
613.3e Layer 7e: Effects that switch a creature’s power and toughness are applied. Such effects take the value of power and apply it to the creature’s toughness, and take the value of toughness and apply it to the creature’s power.