I view polymorph as a necessary change, to get monsters to fight in their own forms rather than changing into a stronger form, which kinda defeats the purpose of using that monster in the first place. For example, an advanced night hag could simply polymorph into an astral deva for the superior stats. Kinda anti-climatic, if you ask me.
Next up, the Ocean Strider.
The art is spiffy, I give you that. It is a giant with a whale for a head. If you have just finished watching Kamen Rider OOO, it looks like a giant version of one of its protagonists, Mezool, making it that much cooler.
Again, it seems the designers were not sure what to make of it. For a Cr18 monster, it has very low AC (19!), attacks (+18/+13/+8), stats (just 19str for a huge-sized opponent?) and hp (only 165). Another interesting thing is that because of its low str mod (+4), it has no chance of taking apart of sailing ship or larger vessel (as the DC is 25 or greater), and faces difficulty sinking even a rowboat. Perhaps it is better for water-based combat than I am giving it credit for; I don't know, as that is not an aspect I am familiar with.
Its updated entry gives it whirlwind, but again, with its crap attack bonus, it isn't hitting anything.
On the other hand, it does have an impressive array of SLAs (elemental swarm, horrid wilting, SNA9, cone of cold and acid fog), so it is in essence yet another high lv spellcaster masquerading as a credible melee threat. And with so many uses of its SLAs, the combat will be long over before it has exhausted them and finds itself forced into melee.
Recommendation: The cr is probably okay (17-18, comparable to a high lv caster slinging those spells around). If you are just going to use it to spam horrid wiltings and don't plan on ever actually fighting, then you can likely just run it as is, though I suggest doing the following as well.
1) Advancing its stats by 2 size categories (ie +16str, +8con, +5NA)
2) Give it magical mithral scalemail (as suggested in its entry)
3) Equip it with a magical huge falchion to further boost its attack bonus. It is tempting to add the skillful weapon property, given the fey HD's poor bab, but do so at your own discretion.
4) Swap in quicken SLA: cone of cold. If you are feeling particularly nasty, maximize or empower SLA: horrid wilting could work wonders as well.
5) Contemplating whether to limit how often it can spam its SLAs, so as to give it some reason to actually enter combat.
Assuming +5 mithral scalemail, this improves the Strider's AC to 33 (base10, +9AC, +1dex, +15NA, -2size). Its hp increases to 285, and its attack bonus rises to +31/+26/+21, 3d6+23 (still not that desirable, but at least it has a decent chance of hitting things). With a +12 str check, it now has a fair chance of succeeding at the tasks its entry lists.
It's poor fort save is another issue (now+16), may want great fortitude as well.
Next up, the megapede.