It's actually one of better balanced mechanics in this game and that's why people who are being used to get everything basically for free (like best armors, gold, ships) are having unpleasant surprise - why is not boarding also that easy?
The biggest factor that is deciding about result in your situation is that super low amount of crew - trust me ~30 people is like nothing, specially when enemy ship have them like 2x-3x more probably. Also your crew equipment is mediocre at best and only in case of swords, while pistols are trash and there are no armors and healing items for your crew at all - how can you know what's the armory of enemy ship? Maybe it's far better than yours. Then its also important how big are skills of enemy captain and which perks does he have - maybe he is much stronger than you. Also "high" morale doesnt really mean THAT high - there are few higher morale levels - you should aim for superior / heroic morale. Last point may be that you only have 85 HP
- dying from 2-3 hits by having 85hp doesnt mean dying fast - you are just weak.
The amounts of people fighting during boarding, their HP and DMG are truly symbolical. This is just best what original game and mod creators were capable to achieve to represent real chances to win or lose during boarding combat.
Now imagine that your crew of ~30 people that are armed, commanded and educated similar to II WW russian red army solders that were capable only of murdering and raping innocent people is attacking an group of maybe ~50-70 people armed far better, with much higher morale that indicates their will of survival and protecting their beloved captain and also commanded by better captain than Ciri because i dont believe that she got any education about leading a ship during her time in Kaer Morhen.
It's something very good and realistic that you are losing during boarding - it's one of not that many really good balanced things in New Horizons mod. You can get unlimited amounts of money and items basically for nothing, you can become a hero of carraiben just by reminding people that they should hide their purses with money, but you CANNOT just grab someone's ship and do with him whatever you want as long as you are not truly stronger than him - at least that thing is balanced pretty well.
- What can you do to grab that ship right now? Your best chance is to lower it's crew by using grapes to absolute minimum value OR fighting with that ship to the point when it surrenders and then you will need to fight only enemy captain or maybe he will also surrender to you instantly.
- What can you do to increase your boarding potential really fast? Get around 20 fine swords (worth atleast 2000 gold each), 20 pistols (also around 2000 gold worth) upgrade them to excelent quality by blacksmith. For upgrading pistols you can go to Sao Jorge in La Grenada - gunsmith lives there. You also should put there atleast few dozens of best healing items you can get and around 10-20 best armor pieces you can financially afford. At the end you should get Nickolas Sharp sword for yourself from side quest and ofc also upgrade it to excelent quality, while very good pistol for yourself you can find on the same island from Clint Eastwood - jungle around pirate settlement.