It used to be hard to win in anything but red or yellow last summer, I remember... heard it was super easy b4 where u could just sail from the harbor til the end and the forts would barely attack you. Then I think around May, they changed it made the forts do tons of damage and rarely was anyone able to win in Green in like June/July. Then when SvS got released end of July, its been the same ever since then. The addition of SvS brought the whole armor shield hull plating thing which made either the Pearl having stronger hull or that the forts and ships barely do any damage anymore, not sure which it is.
I've done it solo probably around 20x or so since the addition of being able to redo BB whenever u want to now, tho I hate u have to bring someone with u lol, oh well doesnt hurt to have a cheerleader hehe orrr someone to use as bait rofl

Out of all the times I've sailed & gunned it solo, only 3 were green wins, everything else was yellow or red...& the 3 green ones were barely even green, i got 49% twice & a 44%, 50% would've made it yellow, so blah.
I think it gets dumbed down by how many ppl attack...ships+forts fire back more often and do more damage at the rate sort reciprocal to how much they themselves r being attacked. Try getting a full crew of people but tell no one to attack... hence why its better to get high lvl gunners instead of low lvl gunners coz low lvl attack a lot which makes the pearl get attacked more but the low lvl gunners r barely doing any damage with each hit, so kinda pointless. But yea the bigger the crew the tougher it is but only if everyone in crew attacks...haha somehow I always end up with at least 1 person who runs wild on deck and cant figure out left from right and doesnt fire a cannon at all lol.
Yea the ideal crew is you + 4-6-8 gunners.
And you dont have to have take cover to be successful lol. The high lvl gunners you had probably took over the lack of cannon power from the lower lvl gunners. I did it with my pirate that had lvl 5 sailing & I'm glad I had the gunners that I had