I have not done undersea plants with UPLC ACCQTAG, but I have looked at plant extracts from tobacco, barley, and wheat. These were water extractions of dried, ground up plant material. The toabacco extraction, for example, was 1 gam of pulverized tobacco extracted in 20 mL of water. The extraction was sonicated for 15 minutes and filtered. You will definitely want to use the ACCQTAG Ultra method for this work. It has a ten minute runtime with resolution better than 30-50 minute methods. I have used the ACCQTAG Ultra method designed for cell culture analysis when working with plant extracts and have not experienced significant interferences from other peaks. Methanolic extractions will result in more interfering peaks. In most cases, the only other sample treatment required is a dilution of the extract 10 - 20 X, depending on amino acid concentrations. Fish extracts have been done successfully to measure biogenic amines as well as amino acids. I imagine the fish extracts might be more messy with extraneous peaks than plant extracts.
ACCQTAG Ultra is a complete method with derivatization chemistry, a method qualified column, and tested gradients for hydrolysate amino acids as well many free amino acids such as found in cell culture media and plant extracts. It uses the TUV or the FLR detector.