^ I am a very pear-ish pear with fat legs–a sheath would look, oh, I can’t even imagine how bad, on me. So, not ALL body types.
I think a sheath is the most difficult to wear and is not particularly flattering on several body types. A sheath looks best on a slim and narrow-hipped figure. An hourglass or curvier figure can only get away with it if the dress has princess seaming. It can also look good on someone with a bit of a tummy or not much waist, but with slim hips and thighs. It’s the worst style for someone with a small waist and generous hips and thighs - accentuates all of the negatives and does nothing for the positives. A shirtwaist or fitted waist and fuller skirt are much better.
I look best in a sheath. It is actually the only shape dress that looks good on me. The wrap dress which so many rave about looks absolutely hideous on me. I am as Gourmetmom described-slim hipped, slim thighed with no defined waistline. I also have a tummy.
I think sheaths look fine on many other people. Just not on me. 
I have an operatic ribcage with no waist, no hips. Even when I am at a good weight things that fit my chest are huge on my hips and legs. I am pretty much an apple. Constructed pants that fit my waist tend to balloon out like the little Dutch boy’s below due to slimmer hips and thighs, and it doesn’t change a bit if I am at my slimmest, therefore I wear pants with stretchy waists that drape well below. I also have shortish legs, so I wear petite pants, but my torso is not proportioned for petite dresses. All of those things that say they are for “curvy” women are horrendous on me.
I haven’t looked for a dress for ages. If I did, I’d probably look for a wrap. I usually go for separates, and I tend to like things that are not shapeless, but have a little flow and drape well.
I completely agree with Gourmetmom here: sheath and wrap styles work well for * certain* types of bodies. I would not look good in a sheath, but wraps are my go to dresses both for work and for play. Since the OP said that sheaths were right up her alley, it sounds like she will be fine picking one out of her closet.
Dressy resort wear for me is flowing draping pants and colorful flowing top set with shell and drapy jacket or top. For fall the colors would not be the pastel or spring colors but something with black in it along with other colors and dressy sandals. DH would wear an Hawaiian type shirt over black slacks. If jackets were called for he would change that to a collared shirt with sport coat.
Dressy resort would generally mean at minimum a sport coat for men. I would think resort casual would imply the “Hawaiian” shirt/pants combo.
@Consolation --we are such opposites! Anything I buy that doesn’t stretch horribly over my hips and thighs, sags like an empty sack over my upper torso. Overall, I think sheaths work best with “straight” bodies.
I don’t have a “straight body,” mine is more like an hourglass and I look amazing in sheaths and wrap dresses (have been told this by many–friends, relatives & strangers, so not only my personal opinion). I think it depends on the individual.
It’s kind of interesting–we rarely receive invitations that mention attire other than “aloha attire” or totally silent. Aloha attire is men wear an aloha shirt and women wear slacks with nice tops or business casual attire. Maybe we’re not in the “black tie,” tuxedo set or clothing in HI generally just isn’t as big a deal. Even at the opening night of the opera, there’s the full range from t-shirt and jeans to ball gowns and everything in between. In HI, men can go pretty much everywhere in an aloha shirt and khakis–opera, office, most weddings or funerals. Women have more nuances and agonize more.
@HImom --well, at least balanced top and bottom–which would include hourglass. Oh, and decent legs. Mine don’t look good in anything that doesn’t totally cover them.
Here’s the Nordstrom’s suggestions about different types of dresses for different body types. @Garland, I think of “straight” as a rectangle, with no definition between shoulders, waist and hips, which is obviously not what you or I agree looks good for sheaths. 
Anyway, here are Nordstrom’s suggestions:
ha! the dress for “pear” is pretty much exactly my MOB dress.
When in doubt I reach for my “little black dress” as I would rather be a touch overdressed than under. Dressy resort wear sounds like something you’d wear on a cruise ship! October in Florida tends to be a hot month so I wouldn’t dress too warmly. I don’t do hose but very sheer black hose with the black dress would maybe make you feel more covered up for the time of year.
Okay, I am none of the Nordstrom’s shapes.
Re pear shape and sheath dresses: sorry to bring up Michelle Obama again, but she’s pear-shaped and not skinny (but in good shape) and she looks GREAT in sheath dresses. I think it is a much better look on her than the full-hipped circle skirts she sometimes wears.
My stomach is a major problem in a sheath or a wrap. They look terrible on me. I don’t have the chest or hips to counterbalance. Legs and arms are fine.
Michelle is tall, does not have a big tummy, and has incredible arms, so these features definitely help. Also, the sheaths she usually picks are not what I think of as a classic sheath, but rather fitted versions that enchance her curves a bit. She definitely knows how to dress to look her best!
HImom, thanks for the link. Very helpful.
Michelle’s dresses are custom fitted - if she bought off the rack she would find most dresses that fit through the hips and thighs would be too big on top. I think she looks best in a fuller skirt with fitted waistline - I also think she looks great in empire style gowns.