Sacred Stones, compared to most of the other games, is very easy even without its grinding enabling. Though this is relative. Sacred Stones splits at the roughly two fifths mark of the game, then comes back together for about the last quarter, where it follows one of the two main protagonists, either Princess Eirika, who is an naive idiot blue headed cabbage of a woman, or Prince Ephraim, who is more protagonist material and has better chemistry with the main villain because he doesn't almost fuck over the whole world because he wants to have sex with him, unlike his sister. Ephraim's the one with the ship level, and his campaign is somewhat more difficult in general, and yes, this is where L'Arachel joins on that route, since she appears in both.
The thing that makes the ship level uniquely fucky is actually very easy to explain. You're locked on a strip of long and relatively narrow ship terrain being swarmed with flying monster enemies with a lot of movement, of both magical and physical attack types, from fog of war. Since you're also being boarded by another ship full of monsters this means you're being surrounded and engaged from every direction from fog like no other level in the game. In basically every other level you're able to channelize and bait the enemy reliably, but not that one.
Then when L'Arachel sails her boat up she's on the other side of the monster ship from you, so her Berserker bodyguard might not be able to protect her sufficiently.