Though I am not working on sea grasses but I have colleagues working on seagras in lab.
I would like to tell you if you need to do barcode or phylogeny, there are two good possibilities to preserve samples depending on location of study site.
If it is near to field station or laboratory.. you can bring seagrasses submerged in seawater and as soon as you reach lab, clean the leaves, flash freeze and store in -80 freeze.
If it is not possible to freeze soon, you can either use 100% mol grade alcohol to preserve or silica gel to dry the tissue.
In this way you can protect DNA from degradation, if DNA is good you can do phylogentics or barcode.
what i want to tell you is that .... bring seagrasses and clean with fresh water and made it dry then take a small plastic bag with silica gel and store it in -80 freeze
what i want to tell you is that .... bring seagrasses and clean with fresh water and made it dry then take a small plastic bag with silica gel and store it in -80 freeze