1. On vacation I played splashing at the sea.

2. On vacation I played splashing on my friends at the sea.

3. On vacation I played splashing to my friends at the sea.

4. On vacation I enjoyed splashing at the sea.

5. On vacation I did splashing at the sea.

(Are all correct? Which one is commonly used?)

Rather than saying "at the sea," it would be better to say "at the seashore" or "at the beach" or "in the ocean."

"I played splashing" is rather awkward. It'd be better to say, "I played and splashed..."

Instead of "did splashing," you should just say "splashed."

We would usually say, On vacation, I enjoyed splashing my friends "at the seashore."

Or, "in the ocean".

Out of the given options, the most commonly used and grammatically correct sentence is:

"On vacation, I enjoyed splashing at the sea."

This sentence effectively conveys the idea of you having fun while splashing in the sea during your vacation. The other options have some grammatical errors or awkward phrasing.

1. "On vacation I played splashing at the sea." - This sentence is grammatically incorrect. The phrase "played splashing" does not accurately describe the activity of splashing in the sea.

2. "On vacation I played splashing on my friends at the sea." - This sentence is grammatically correct but might not accurately convey what you meant. It implies that you were splashing water on your friends rather than simply enjoying splashing in the sea.

3. "On vacation I played splashing to my friends at the sea." - This sentence is grammatically incorrect. The phrase "played splashing to my friends" does not make sense. Instead, you could say "On vacation, I played and splashed with my friends at the sea."

5. "On vacation I did splashing at the sea." - This sentence is grammatically correct but sounds less natural compared to the commonly used sentence mentioned earlier. It is clearer and more concise to say "On vacation, I enjoyed splashing at the sea."

Overall, the sentence "On vacation, I enjoyed splashing at the sea" is the most commonly used and accurately expresses the concept of having fun while splashing in the sea during your vacation.