Starting Issue-stumped

Hey guys, today I experienced a starting issue for the second time and it has me concerned. 3 weeks ago I was out with a buddy and had shut the engine down to put the barefoot boom on, when I went to start the boat back up it seemed like the battery was dead. I have recently put in a new Optima red top. My voltmeter consistently reads at 14 volts unless I have the bilge and radio on at which time it bumps down to around 13. Normal I would think. It barely would turn over. I waited a few minutes and it sluggishly started cranking again, then cranking faster to the point where it fired up. I was like WTF? Ran fine. We stopped at the local island on the lake and I turned her off, let her sit for about an hour and when we left she fired right up. Fast forward to today, went out w/ some friends, fired right up at the ramp, put in and we went out. Turned off a few times while we changed skiers, went to the island, no issues. At the ramp I left the boat off while I went to get my truck. When I hopped back in to put it on the trailer I experienced the SAME EXACT THING as the previous time. whaaa whaa whaaaaaaa. ??? turned key off, tried again. whaa whaa whaa. turned off, tried again....whaaa whaaaaaaaaaaa, slowly started cranking faster so I stayed in it and it cranked fast enough to fire and I put on the trailer. Could this be my starter? Alternator? Boat is in my driveway now so I was thinking about pulling the alternator and taking in to get it tested tomorrow.

Corrosion, clean all battery cable connections. Could be corroded cables in which they will need to be replaced. Check voltage at starter next time when dead if possible. Basically back track to find voltage drop (resistance). Since you've recently messed with the connections though, that's probably it.