I went to the beach today but didn't get in the sea because it wasn't calm at all. I want to tell my friends that speak English about it and I wonder how should I tell them that. Should I just tell them that the tide was high or is there a better word to describe it?
In common speech, the opposite of a "calm sea" is a "rough sea". If it's very rough we may say "turbulent". Another word for "calm", in the context of seas, is "smooth", by the way. If the water is rough and it's also raining, you can say "stormy seas".
Note that saying the tide is high has nothing to do with being calm or rough. "High tide" means that the water level is high, i.e. the water is deeper and/or comes farther up the shore, due to the tidal actions of the sun and moon. You could have high tide and be completely calm or low tide and be completely calm, or high tide and rough or low tide and rough.
We usually call it a wave which means:
A long body of water curling into an arched form and breaking on the shore
If you say, "The waves were (very) rough today", it would get the meaning across.
You can replace "rough" with "high".