Based on the description in your question, I can recommend two ultra-thin, waterproof portable Bluetooth speakers that I think have good sound quality. You can refer to the following:
Philips BT3900? ? This Bluetooth speaker It is an ultra-thin and portable model. The body is ultra-thin and lightweight and can be carried in your pocket. The speaker has a sleek and stylish appearance and feels great in the hand. Let’s talk about the sound quality of this speaker. The sound is very clean and delicate, without any noise, and it is comfortable to listen to. In addition, this speaker has an IPX7 waterproof function, so it can be carried safely when going out.
This Bluetooth speaker from Wecele? You are left and right. The design of the speaker is very professional and you have the choice between 4 colors. The sound quality of this speaker is also very good. The ultra-thin case has twospeakers designed to form a stereo channel. The listening atmosphere is particularly good, the sound is clear and delicate, and the midrange and treble ductility is. also particularly good. In addition, the speaker also supports pluggable card reading and can also be used as a power bank. It can be said that this is a very cost-effective little speaker. The speaker also supports IPX7 waterproof feature, making it safe to carry when going out.
The above is for reference only, hope it can help you~