Glucuronyl hydrolase involved in ulvan degradation. Ulvan is the main polysaccharide component of the Ulvales (green seaweed) cell wall. It is composed of disaccharide building blocks comprising 3-sulfated rhamnose (Rha3S) linked to D-glucuronic acid (GlcA), L-iduronic acid (IduA), or D-xylose (Xyl). Unsaturated 3S-rhamnoglycuronyl hydrolase works together with ulvan lyases to fully degrade the ulvan polymer, catalyzing specifically the cleavage of the unsaturated 4-deoxy-L-threo-hex-4-enopyranosiduronic acid (deltaUA) of the deltaUA-oligosaccharides deltaUA-Rha3S, deltaUA-Rha3S-IduA-Rha3S and deltaUA-Rha3S-Xyl-Rha3S, the end products of the ulvan lyase reaction. Belongs to the glycosyl hydrolase 105 family.