Supply cannot be traced across All-Sea Hexsides.
Maybe it doesn't have to trace at all:
12.6 Ports and Coastal Hexes as Supply Lines
A supply line may trace from one port to another port on a
friendly-controlled sea as part of a river segment. Units on a
coastal hex of a friendly controlled sea are considered automatically
in supply, even if there is no friendly controlled supply
source on that sea.
By the same token, a unit doesn't have to move across all-sea hexsides because it can make:
10.5 Sea Strategic Movement
To conduct sea strategic movement, a unit moves from one
friendly-controlled port to another friendly-controlled port
within one sea. Units may not move from one sea to another.