Washing or rinsing fish and seafood
Like in all things: IT DEPENDS and always try to buy the freshest seafood as possible and consume ASAP.

Fish, depending on the fishmonger, sometimes I will rinse a whole fish it if there is blood or innards still in there; I will make certain after to dry it out completely before using.

For filets and portioned fish or shelled shrimps, no, i will only dry it out.

For shellfish like mussels, clams, I will rinse them and let them soak in cold salted water (I never tried the corn meal trick, google it).

For raw unshelled shrimps, I will clean them out and rinse after.

For your question about Ceviche, I will use the above guide (my own) to prepare the fish before cutting and marinating.