Is it safe to autoclave seawater agar?
I see no reason why it would precipitate chemically. Besides, you need to dissolve the agar via heat. Hence, auclaving is fine. 

If you do not believe me these protocol allow for autoclaving of seawater agar 15min cycle 121 degrees: 
1. https://www.dsmz.de/microorganisms/medium/pdf/DSMZ_Medium246.pdf
2. http://himedialabs.com/TD/M592.pdf