I am a fisherman. I want to know the weather on the sea. What websites are there?

Weather forecast of offshore area of Central Meteorological Observatory;

/publish/marine/ offshore.htm

There is also a large-scale website including sea surface weather forecast, which belongs to meteorological professionals and the forecast time is as long as 15 days. It might suit you.

Forecast precipitation:

/gfs/frame/prec_frame.htm

Predicting sea level pressure:

/gfs/frame/pslv_frame.htm

Other contents can be viewed by yourself as needed.