Relax

Last weekends I went to visit my friends in the who live upcountry and we went to the sea.
When we got there, everyone jumped to into the sea and swimming swam, but me I just played in shallow water because I can not swim.
This time is the low season so I saw not did not see much many travelers. If you guys would like to visit the sea in my country. then I do recommend to travel in from December to May.

Welcome!

   = suggestion; it is correct but I would not say it that way.

   = pluralization error.

   = tense error.

Last weekends  I went to visit my friends in the who live[1] and we went to the sea.
When we got there, everyone jumped [2] to into the sea and swimming swam[3], but me I [4] just played in shallow water because I can not swim.

This time is the low season so I saw not did not see much [5] many travelers. If you guys would like to visit the sea in my country. then I do recommend to travel in from December to May.

[1] You could say "... who live upcountry ..." which means that your friends live in the "upcountry", or you could say "... visit my friends upcountry ..."

[2] Because you used "got", it means that the action happened in the past so the other verbs (like jump) have to be in the past as well.

[3] Swam is the past tense of swim. You could also say "and started to swim".

[4] Use "I" when the subject is you. Use "me" when you are the object. For example, "I talked to him" ("I" is the subject and "him" is the object) versus "He talked to me" ("He" is the subject and "me" is the object).

[5] Use "many" when the object can be counted. A guideline you can use is if the object (tourists) is plural then use "many". If the object is singular use "much".

Your English is very good!