With "since", I can talk about 1) something which started in the past and which continues up to the present, 2) something which happened one or several times in the past.
1) I have worked for this company since 2015. (I have been working for this company since 2015.)
2) I have swum in sea since 2015. ( I have swum once or twice since 2015. It is not meant for continual swimming. )
Am I right?