Please read this whole issue, as every detail is relevant.

Anyone riding a boat, including the host, LAN connected and externally connected players, causes huge server lag issues.

The issue, in summary, causes the server to give a WARNING "server overloaded" error that are approximately 50 seconds apart and cause the game to pause for all players for about 5-6 seconds each time. The problem persists until all boats are dismounted.

The issue is not present, or at least not obvious, when riding horses, even very fast ones. This was confirmed while riding 4 horses simultaneously across some distance. However, even a player IDLE inside of a boat will cause said server overload issues. Speed, and even if the player is moving, is irrelevant.

The following is an outake of the server log demonstrating the issue.

[17:15:45] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 5790ms behind, skipping 115 tick(s)
[17:16:34] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 5775ms behind, skipping 115 tick(s)
[17:17:23] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 5862ms behind, skipping 117 tick(s)
[17:18:12] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 5822ms behind, skipping 116 tick(s)
[17:19:01] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 5837ms behind, skipping 116 tick(s)

The error had not happened until version 14w11b. However, we rolled back to 14w10c and the error persisted (it may just not have been noticed before). Since the conversion to UUIDs took place then, it would not at the moment be practical to test if this error happens in previous versions though I will likely be doing some tests tonight and update this post if so.