This isn't how your immune system works. Your immune system builds resistance to every pathogen that invades your body, if you survive the infection. This immunity is pathogen-specific; memory cells that have learned to effectively kill pathogenic E. coli, a bacterium you probably frequently encouter if you 'dive in sewage water', will do absolutely fuck all against SARS-CoV-2. Memory cells that are specific to normal human coronaviruses may offer some sort of limited resistance, but no immunity, and memory cells that aren't specific to any coronaviruses will not help whatsoever, and you might as well not have any memory cells at all. The only memory cells that can effectively make you highly resistant or immune to COVID-19 are those that have become specific to SARS-CoV-2. The memory cells you've developed to pathogens in sewage are specific to several species of bacteria, which offers no help to any viral infection.
In children, I agree that they have to get dirty sometimes and there is such a thing as being too clean, as in those early years the immune system needs stimulation and exercise to mature and promote a healthy relationship between the immune system, the bacteria inside of you and the bacteria in the environment. But in adults with a mature immune system, higher-than-average exposure to pathogens has little effect. Of course, adults do need to come into contact with normal circulating pathogens to maintain resistance to them, but the benefit exposure brings peaks around this normal level. At higher levels than this you are just putting your immune system under extra stress, which will decrease its ability to fight in the long run.
The best ways for an adult to maintain a healthy immune system, both innate and adaptive, is through exercise, adequate nutrition and avoiding as much psychological stress as possible. With COVID, your best defence is your innate immune system, which covers physical barriers such as mucous and epithelial cells that line your respiratory system, and also the innate cellular response such as inflammation and immune cells like macrophages that attack anything that appears to be foreign, dying or neoplastic, so exercising and generally keeping healthy is important during this pandemic.
But back to the point, refusing a free or very cheap vaccine because 'I haz a stwong immune sistem' is a ridiculous and infantile argument. You have a strong immune system? Great. The vaccine will be more effective for you because your adaptive response will be stronger and the memory cells you build from the vaccine will last longer. That also means you can't pass the disease onto those with weaker immune systems than you for longer as well. The point of giving young people the vaccine isn't to stop young people from dying, because obviously they don't die from COVID, but to halt the spread of the disease so vulnerable people who might actually become seriously ill from it don't come into contact with it from younger people who have no symptoms and have no idea they are passing the virus on.
In the same way swimming in sewage water doesn't protect you from measles, swimming in dirty water doesn't protect you from COVID-19 either.