This is not necessarily true. While you may get more xp per kill if you use lower level items, you will get kills significantly slower. It's like this - if I can get gas that will allow me to drive 25 miles/gallon but costs $5.00/gallon, it's not really any more fuel efficient than if I use gas that gets me 15 miles/gallon but costs $3.00/gallon, because after driving 75 miles I'll still have spent $15 on that gas. This is why leveling with grenades isn't always unbearably slow despite the low amount of hits per kill - you get a higher quantity of enemies defeated, so while your xp/kill goes down, your amount of kills goes up due to the high damage of the grenade as well as its AoE type damage. It is also what makes leveling with broadswords go so quickly - you get pretty good marks in all the categories. High damage, high amounts of hits/kill, and AoE damage so a high amount of kills overall.
As for sailing leveling slowly, I would agree that it's not the quickest thing to level. However, personally, I think that it balances out at least a little. You see, when you're on land, your rep is split between 6 different weapon types - or if it's not, then for every weapon you're not using, you're not gaining any levels for that weapon, which still hurts you in the long run. And while land combat is arguably most of the game's content, or at least 50% of it, the other huge chunk of content is purely sea combat. Sea combat is split into just two weapon classes, sailing and cannon. So you may be leveling individual weapons pretty quickly in comparison to sailing, but leveling the 6 weapons overall will happen at a pretty comparable speed to leveling sailing and cannon. Still, though, sailing does level much slower than cannon, for sure, as cannon can get in tons more hits than sailing. I'm not sure if your sailing rep is affected by anything more than broadsides and the type of ship you sink, though, so other skills might also play into it unless someone can confirm they do not. But in my mind the major inequality is mostly the difference between the leveling speeds of cannon and sailing, not sailing and land combat skills.
Hope all that made sense.