Short OVA's all seem to suffer from the same issues for me personally. And that is they never give the characters or story enough time to develop, I never grow attached to the characters or care about what is occurring with the story. And Blue Submarine No. 6 is no different in these same ways. Everything from episode 1 feels incredibly rushed with no time dedicated to the characters or world.

The characters all seem to have the same style backstory, tragic past involving dead family so they wish to kill the sea monsters. That is it, that is everyone's main story in a single sentence. Even the main villain has a very similar backstory. It makes it all feel depressing sure, all these people on a quest of revenge and survival against these monsters. But at the same time makes it very shallow, you can't build an entire cast on the same basic idea and definitely not in 4 episodes. A couple characters get some very basic character development, but nothing I could consider incredible. The main development would be Hayami Tetsu, the mysterious Ace pilot who becomes curious about the monsters. He still gets the tragic backstory like the rest, but is given a bit more development to flesh him out a bit more. The other major development is Kino Mayumi, whose main development seems to be overcoming a pursuit of revenge and falling in love. A love I have to mention that seems to be the most shoehorned thing in history, you can't make me feel like characters fell in love with a sentence here and there. Verg the enemy leader gets last minute development, and I mean last minute, his development happens in the last 5ish minutes. Myutio a female sea monster gets her 5 minutes of development as well, helping Hayami and maybe making him understand them better, I think that was her point at least.

Now I want to mention the main villain, Jung Zorndyke. Well I don't know if they were actually trying to make him a true villain, more like a misguided man who wanted to be a Hero in his own twisted way. He thought be doing his actions he could save humanity and make the planet a better place, and if that failed allow it be given to something new.
I feel like this falls apart though once you look at the way he behaves

The animation in the show I felt was decent enough, as long as they didn't use the CGI in the scene. But that was apparently a very difficult thing for them to achieve. The amount of very poor and horrible looking CGI reminds me of pre-rendered PS2 cutscenes. With the fps seemingly in single digits at times, and very low art quality on the CGI. Overall all the terrible CGI just takes away so much from scenes that might have been cool. A decent underwater fight becomes something you can't help but laugh at when viewing these CGI blobs move around the scene. I wish I could view this 20 years ago, and see what I would think of it based on other stuff available. But unfortunately that isn't possible, and I have to look at it as somebody who can't ignore bad CGI no matter the year it was created.

The music was pretty good, felt like it did a good job. With different styles depending on who was on the screen at the time. Made the music feel like it was flowing well with the scenes and what was occurring.

The Story wasn't incredible, kinda felt like a Climate Change story before it really was a major theme in shows. A flooded planet due to melted ice caps, and genetic engineering changing the planet to fit our needs being bad. Felt like it was kinda weak in the end, with just not enough time put towards developing the story alongside the characters. Instead a bunch of story is given in the last episode by the main villain and didn't impress me.