Thats how I've been using them as well (without guiding eye character). It has worked pretty decently so far.
I run the unit at 30 (10x3) with the sea serpent standard
If you move the unit into range it will get 42 shots hitting on 4+, then when the enemy charges you get 42 more shots hitting on a 5+, and then 41 re-rolling to hit attacks usually before the enemy can strike due to higher I.
According to maths against a t3 unit with a 5+ save you should do at least 20 wounds before they even strike a blow. And with so many dice being thrown there is potential for many more. This means you can pretty much wipe out an elite t3 unit eg. swordmasters or black guard or you will remove at least the last 2 ranks of a larger unit probably removing steadfast)
plus there are many available synergies with spells. If you have a nearby sorceress casting soulblight then the carnage massively increases. Or what if you made those 41 re-rolling attacks s8 with Okkams mindrazor
It has done pretty well for me so far
- units with 10 wounds or less eg. fleshounds, knights may as well not bother going near it as they will get raeped
-light infantry units get torn up by it
-with flame banner it has blatted an Abomination in one go
What it isn't good against is large units of tough and well armoured troops like chaos warriors. They will run in laughing as the bolts patter off them like rain and then will tear your corsairs to shreds.
-Also having frenzy means there is a tiny chance the unit will rush into combat without you getting to fire at all. This has never happened to me when the unit has been near the general and BSB. It did happen once when they were out of range however and caused them to get creamed by a large halberdier block. I like the 10 extra attacks but some people might see this as an unacceptable risk and take AP banner or flame banner instead.