The ornamental design for a boatseatbeverageholder, as shown and described.
FIG. 1 is a perspective view illustrating a boat seat beverage holder;
FIG. 2 is a side view of the boat seat beverage holder;
FIG. 3 is a top view of the boat seat beverage holder;
FIG. 4 is a bottom view of the boat seat beverage holder; and,
FIG. 5 is a perspective view illustrating the boat seat beverage holder secured to a boat seat.
The broken lines in FIG. 1 through FIG. 5 are included for the purpose of illustrating the environment in which the article resides and form no part of the claimed design.