Hand axes say hi.
Yes, as FE3 Player has suggested, let's look at Dart's hit. A sampling of Ch 19's enemies:
Cavs:
3 w/ Iron Sword--16 Avo
1 w/ Iron Lance--16 Avo
1 w/ Steel Sword--14 Avo
1 w/ Steel Lance--8 Avo
Nomads:
7 w/ Iron/Short Bow--20, 22 Avo
3 w/ Steel Bow--12 Avo
For the record, Hand Axe Dart doubles none of these, not even the Steel Lance Cav. Hitrates for Dart:
Iron Sword Cav: 43%
Iron Lance Cav: 88.7%
Steel Sword Cav: 46.6%
Steel Lance Cav: 95%
Iron/Short Bow Nomad: 60-63%
Steel Bow Nomad: 73%
Granted that he can use the Iron Axe against Nomads on player phase, which helps him a good bit. Iron Axe hitrates:
Iron/Short Bow: 82-85%
Steel Bow: 92.2%
Anyways, Dart's average for the Hand Axe against all these enemies is 63% real. I'm gonna say that's pretty shitty. Meanwhile, a L11 Lucius has 99.55% hit against the 22 Avo Nomads, the most evasive enemies on the list.
Let's also keep in mind that this chapter has alot of forests. What if the enemy lands on one? Hitrates for Hand Axe Dart against enemies on forests:
Iron Sword Cav: 13.8%
Iron Lance Cav: 61.2%
Steel Sword Cav: 16%
Steel Lance Cav: 74.4%
Iron/Short Bow Nomad: 25-28%
Steel Bow Nomad: 37.1%
Overall average of 30.6%. Ouch. I won't bother to show all the numbers for Lucius, but he has 88% against 22 Avo Nomad + Forest, so yeah.
So Dart can either take counters, or lose offense by even more, since he's already losing even if he uses Iron/Steel; L11 Lucius has 13 Spd to base Dart's 8.
It's not that big of a deal since magic users are rare and spirit water poses an easy solution anyway.
Pure water is a good point, but magic enemies actually have quite a presence in some midgame chapters. Ch 21 features significant numbers of Monks, Ch 22 has 11 Mages, 2 Shamans and 6 Monks, Ch 23 has Mages near your starting point (and actually a large number of magic enemies in general, but Pent kills most of them), and 23x is obvious.
Dart can also walk on water and has a ~30-45 crit advantage on Lucius.
At 20/1, Lucius gets 2 extra crit over Dart from Skl, another 5 from a B with Raven, and 8 from Shine, canceling out Dart's class bonus. Dart's crit advantage is 30 if he uses Killer (or I guess 33 whenever Lucius uses Lightning, w/e). Let's look at Ch 24, which is usually regarded as promotion time. There's 2 main types of enemies here, Mercs and Wyverns.
24 Avo is a common value for Mercs. Hand Axe Dart has less than 50 hit so I'm going to ignore that. Killer Axe Dart has 54.4% hit, which is better but still pretty bad. Lucius + Lightning is 2HKOing and doubling with 100% hit.
Killer Axe Dart indeed 2HKOs Wyverns without any significant hit problems, but Hand Axe Dart is another story. Hand Axe is very borderline for a 2HKO; on average it fails to 2HKO the toughest Wyverns. Likewise Iron Lance Wyverns can go up to 18 Avo, giving Hand Axe Dart 93.49% hit against them, which is about 11% odds to miss on a double.
Promoted enemies in this chapter are Heroes. Most notable is the L10 Hero with a Silver Sword; guy has 47 Hp, 16 Def, 8 Res, 34 Avo. This guy is so beast that he actually has crit on Dart; 9 crit vs 20/1 Dart's 7 Luck. Even if you risk attacking him with the Killer Axe, Dart still has a depressing 35.7% hitrate, and the Hand Axe is even worse, so really Dart might as well not bother. Lucius + Shine deals 26 damage (a 2HKO) with 100% hit and 21% crit, and his Raven support gives him enough CEV to avoid getting critted, so he could actually beat this Hero 1-on-1 with an enemy phase counter followed by finishing on player phase.
Dart will presumably do better later, as he eventually passes Lucius in Spd and Swordreaver becomes buyable, but I certainly think Lucius wins up through Ch 24 at the minimum. And there's still CoD which is nearly an auto-win for Lucius IMO, so even if Dart wins from 25 through the final, that's 7 chapters not counting the final, vs Lucius winning for 10 (counting Pirate Ship in Lucius's favor). And I'm not entirely sure if Dart clearly wins later on, or by how much compared to Lucius's earlier wins, since any lategame victories for Dart will be based on durability, which is countered by staves.
I might do more detailed stuff later.
As for waterwalk, that's cool for 28x, where Physic is also really cool. Aside from that I don't see any other situations where utilizing it is a good idea. For example, in Ch 25 you can have Dart walk around in the middle of that lake if you want, there's just no point in it.