The Sub-Mariner will most likely be debuting in the MCU during Phase 4. Marvel recently got the rights back to the character, and there was an Easter Egg reference in Avengers End Game. Here are some of the books to consider investing in.
Fantastic Four 4
February 1962. The first Silver Age appearance of the Sub-Mariner. Also the first time Johnny Storm uses the phrase "flame on". There is not a huge amount of difference in sales prices between a 3.0 and a 4.0. Both hover around $2,000. But from 4.0 to a 5.0 the difference is about $1,000, with 5.0's going anywhere from $2,600 to $3,500. 6.0's can go for prices similar to 5.0's, but when you hit 6.5 the price can practically double.
Tales To Astonish 70
This came out in August 1965, so from Sub-Mariner's Silver Age reintroduction in 1962, for about three and a half years he was floating around making guest appearances in different titles. Finally, in August 1965 he received his first solo stories since the Golden Age, here in Tales To Astonish 70. Sub-Mariner gets half of the book alongside the Hulk, and Sub-Mariner stories last for 31 issues from number 70 to 101. This one is criminally undervalued in the 8.0 - 8.5 range. It can be had for less than $200, whereas the 9.0 in recent years has sold for over $300 twice out of the four sales recorded since 2019.
Iron Man and Sub-Mariner 1
This is a unique little one-shot because it bridges the gap between Tales of Suspense 99 and Iron Man 1, and Tales To Astonish 101 and Sub-Mariner 1. It came out a month after the old titles ended, and just a month before the solo titles began April 1968. Captain America and The Incredible Hulk simply took over the old titles and continued the numbering sequence without skipping a beat. So Captain America 100 and Hulk 102 came out in April 1968, but Iron Man 1 and Sub-Mariner 1 did not start until May, so in April was Iron Man and Sub-Mariner 1. Maybe Marvel wanted to give a month to acclimate fans to the new idea before exposing their separate number 1 issues. This is a very affordable book, the 9.2 is around $500, the 8.0 is about $250, and the 6.0 has hardly gotten over $150. A 9.6 is only around $1,300, which is very reasonable compared to Sub-Mariner 1 which would almost be double that price in the same grade.
Sub-Mariner 1
Speaking of which, Sub-Mariner 1, like Iron Man 1, continues the story from the one-shot. It had a good run with 72 issues and is probably one of the more underrated titles. This is a very popular book with everything in the 9.X's surpassing $1k. A 9.6 is about $2,500 right now. An interesting comparison is with its companion, Iron Man 1. In 9.6 Iron Man 1 has been consistently passing $5,000 for several years. Sub-Mariner 1 is half of that currently. However, there are many more high-grade copies of Sub-Mariner 1 than Iron Man 1. I can't help but think that the Sub-Mariner cover's colors do wonders to hide flaws, whereas tiny flaws are much more glaring on Iron Man's deep purple cover.
In Closing
Namor The Sub-Mariner is one of the classic characters in Marvel Comics pantheon. He is not a fly by night character that will go away after the hype dies down. Regardless of movie speculation, this is one of those characters that would be good to invest in for the long haul.
