Why Roman Mediterranean naval power atrophied

Did Roman naval warfare atrophy after the Mediterranean became Mare Nostrum, or did civil war preserve the capability?

It atrophied substantially because Rome lacked a peer naval enemy; smaller forces remained for piracy and region-specific duties such as service around Britain.

Roman control of the Mediterranean removed the rival naval power that had justified a larger battle fleet. Ships and fleets contracted because piracy suppression did not require the same heavy warships. Civil wars are treated as primarily land conflicts rather than a substitute for that absent naval rival. Conditions around Britain posed a different problem: the Classis Britannica used broad-beamed ships to improve stability in North Sea waters. Regional conditions could therefore shape useful vessels even as Mediterranean battle capability declined.