There have been so many examples in using natural biomolecules extracted from Algae as antiviral therapeutics. A group from Zhengzhou University, China, etc. has summarized as a review paper.
https://www.mdpi.com/1420-3049/26/8/2134/htm
Carrageenan, Gaalactan, Chitosan, Agar, Fucoidan, Laminaran are so famous as algae-derived antiviral polysaccharides, and you might have heard about it once. The antiviral mechanisms of these polysaccharides underlie several processes, including the inhibition of viral absorption, the inhibition of virus transcription and replication, and the improvement of host antiviral immune responses. Since polysaccharides are water-soluble and vary safe, it would be quite easy to use as internal medicines. If you are interested in these polysaccharide in more detail, please refer to the original paper and its references.
As you know, since the viral envelop proteins are heavily glycosylated, lectins could be also used to inhibit virus adsorption onto host cells. There have been a number of such examples as shown below.
A past blog article summarizing how lectin-glycan interactions are to be used to combat COVID-19 would be a good reference from a view point of antiviral therapeutics. It would be a big challenge how side effects of using lectins as antiviral ones could be controlled.