Info I found about Riga ports from cruisemapper;
On July 20, 2018, the port received its ever-largest cruise liner - RCI-Royal Caribbean's Serenade of the Seas (length 293 m / 961 ft). The boat moored right into downtown, which became possible after cleaning works (riverbed sediments) conducted in the Approach Channel and the Turning Basin (near Vansu Bridge). Previously, such larger ships were moored at KS Terminal Pier (Krievu Island), where water depth and turning basin's size are proper. After cleaning the sediments, the swinging basin's diameter was extended from 380 to 400 m (1312 ft) which is the max possible in regard to the existing port infrastructure. These works (done by Rohde Nielsen AS) included over 111000 m3 of excavated soil that was used for construction works in Kudziņsala.
So why can’t Voyager dock there?