Herb, you've got a lot of questions here. I'll try and answer a few:
1) "Horse artillery" was artillery (generally light, mobile pieces with all the supporting units (caissons and such) fully mounted) specially designated to accompany cavalry.
2) There were batteries of artillery from the prewar U.S. Army that served in the Civil War.
3) Cavalry units often attacked riverboats and gunboats. Cf. the Battle of Blair's Landing, in the Red river Campaign, and Forrest's 1864 raid on Johnsonville. The actual attacks invariably involved artillery supported by dismounted cavalry.