I'm running through a few simulated/solo encounters in Sea of Blood to ensure I am reasonably comfortable with the rules when our campaign starts in a few weeks and I've run into a couple of issues:

(1) RE: canons - can the heroes spend fatigue to roll additional black dice and/or replace with silver/gold dice like a normal attack? I assume so since it is an attack...

(2) RE: encounters - the heroes seem to be at a severe disadvantage in early combat encounters (1 cannon versus 2 - 4) such that their only viable option is to flee; additionally, moving the ship starboard or port to flee off the side is an iffy proposition without spending fatigue every turn. Add current and rocks and it's a recipe for disaster. Is this "imbalance" corrected as the heroes gain XP and upgrade or is a house rule worthwhile?

I was thinking of adding a house rule whereby the hero could roll one additional white die (in addition to his melee trait dice) when manning the helm to improve lateral movement. However, based on a house rule proposed by a previous poster, I thought I would balance that by requiring that a hero ship can only flee off the far edge of the ocean map; the lateral movement being helpful to steer around obstacles and maintain a safer distance from the overlord ship's cannons while making the run.

This house rule seemed to balance out the encounters during the few solos I ran through with the Overlord still defeating the heroes half the time. But, like I said, I don't want to add a house rule if the issue will correct itself as the campaign progresses.

Any thoughts or advice?

(1) RE: canons - can the heroes spend fatigue to roll additional black dice and/or replace with silver/gold dice like a normal attack? I assume so since it is an attack...

Yes. It's an attack so you get all normal attack rules including appropriate skills and the ability to spend fatigue.

(2) RE: encounters - the heroes seem to be at a severe disadvantage in early combat encounters (1 cannon versus 2 - 4) such that their only viable option is to flee; additionally, moving the ship starboard or port to flee off the side is an iffy proposition without spending fatigue every turn. Add current and rocks and it's a recipe for disaster. Is this "imbalance" corrected as the heroes gain XP and upgrade or is a house rule worthwhile?

I haven't looked at all of the maps, but from my understanding there are few maps that the heroes can't escape easily by spending all of their fatigue to go sideways off of the map. They get it all back when the encounter ends, so it doesn't matter if they have to spend it all every turn the encounter lasts.

The first encounter could definitely be scary, depending on what you pull. The heroes should definitely go to the land-locked dungeon first and then decide if they want to risk it or spend some of that money upgrading the ship.

I was thinking of adding a house rule whereby the hero could roll one additional white die (in addition to his melee trait dice) when manning the helm to improve lateral movement. However, based on a house rule proposed by a previous poster, I thought I would balance that by requiring that a hero ship can only flee off the far edge of the ocean map; the lateral movement being helpful to steer around obstacles and maintain a safer distance from the overlord ship's cannons while making the run.

I'd avoid house rules if you've never played it before, especially house rules that require adding more house rules in order to keep them balanced. Sea encounters are more dangerous than encounters in Road to Legend were, but that's a good thing IMO. RtL encounters usually meant spending a lot of time to get the heroes free money, so much so that the heroes usually hope to roll one unless it's a red trail. SoB encounters can actually cause a TPK, so there no "we hire a guide to help us find encounters" issue to worry about.

This house rule seemed to balance out the encounters during the few solos I ran through with the Overlord still defeating the heroes half the time. But, like I said, I don't want to add a house rule if the issue will correct itself as the campaign progresses.

The Overlord should not win half of the encounters. If he does, the heroes will never be able to get anywhere by sea. There should be risk, but not a major one.