There is one group of people it truly pains me to see in the summer. They don't even make it to the tops of worst dressed lists because it's just that bad. It's possible that no other group of people is worse suited for summer weather than male goths and it breaks my heart. Rather than recommending to dressing like "normal" people, I have devised ten, hopefully helpful, suggestions that fit with the goth aesthetic, specifically for men.
source (note: don't wear black silk in the sun EVER because it will get bleached)
Before I begin, I would just like to say that there are certain materials that you must avoid; leather, wool, velvet, heavy denim and synthetics (like PVC). I understand these are key in a kosher goth wardrobe, but they will make you sweat like a beast. I suggest cotton, linen, satin type materials and silk. Notice the look above. It doesn't look very "summery" but the jacket is made from a light material (possibly silk), so it's not as warm as it looks (by the way, this is an evening look).
My suggestions are as follows:
1. Wear a cotton or linen sports coat (not wool or cashmere, read the tag). You can wear this over a band shirt or a button up shirt. The crucial thing is that you read the tag, it must be made of a light fabric. You can certainly roll up the selves to show off any awesome arm tattoos or bracelets. Go for a slim fit, but make sure that it's not too snug. No matter what the fabric is, clothes that are too tight will make you warmer.
2. No Tripp shorts!!! You can get away with Tripp pants during the winter or whatever, but overly baggy shorts make your legs look weirdly skinny. I don't like shorts on men at all, but if you must wear shorts, choose either (slim) denim cut off shorts (ending just below or just above the knee) or (slim) cotton/linen shorts. You can make these very easily from some old jeans or pants, just make sure not to cut too short. No cargo shorts unless you are doing some kind of outdoorsy activity because they are really unflattering. I might as well note that you should NEVER wear white socks.
3. If you are inclined to dress the neck, choose cotton ties or bow ties. I hate seeing guys wearing tweed/wool ties in the summer. The one below is plain chambray on the outside and pink on the inside (from etsy). Also, you can sometimes wash these instead of dry cleaning, which is really practical.
4. Dress pants made from a light material (cotton), again read the tag might be better. You don't have to wear these with a fancy shirt, just a band-t, but they will be more breathable than jeans.
5. Get a hat (not wool), and certainly not a top hat (definitely not in the day time). I recommend a black straw hat because even a cotton fedora might be too hot. Also, go with a colorful feather.
6. I did a post on sunglasses, but it's worth mentioning again that no look is complete without them, especially in the summer (by the way I know this guy is wearing a top hat but we're going to let this one slide).
7. So, the main things that you should NOT wear are the leather trench coat or the leather motorcycle jacket. Leather jackets are simply not congruent with summer weather. Instead, get a classic denim jacket, in blue or black (or any other color, I guess), or a motorcycle style denim jacket. You can customize it with patches, studs or paint the same way as a leather one but it will be much more comfortable.
8. So, a denim jacket is still not the ideal solution, even though it is better than leather. In my mind the ideal solution is to get (or make) a vest from a denim jacket . Just cut the sleeves off of one you probably already have lying around. Another option is a utility vest if you want a look that's more militaristic. There should be enough pockets for all your goth trinkets and you will still look fully accessorized but you will be much less sweaty. I know this one doesn't have too many pockets but you get the idea (it's on etsy).
9. The other thing that you should not wear in the summer is the leather boots. They look awesome, I get it, but they make feet stink and are thoroughly uncomfortable in the heat. Instead wear canvas shoes like vans or converse or if you must wear boots, find some canvas boots (better than leather with socks (presumably)).
10. Sunblock, do not go anywhere without it, have some in one of the utility pockets or something (this guy can totally be in a sunblock commercial aimed at goths, he is so happy!).
I hope this list has been helpful. The key thing to remember is that you can wear whatever garment type you want i.e. boots, coats, whatever but you need to choose the appropriate fabrics for summer. I didn't mention sheer fabrics or fishnet but those are also good choices (as long as it's tasteful). I didn't mention anything for beachwear either, honestly, you are on your own with that one (remember to wear sunblock). I also like to talk about fragrances on this blog and I think that you should consider a nice cologne specifically for summer (axe doesn't count) so that you smell nice. Of course, these suggestions are also applicable for women and non-goths. Let me know what your fashion tips for summer goth survival are.