RelatedTo does not seem to work! I've looked at several answers on here regarding this one and as far as I can make out this should bring out all entries related to a specific category slug. However it just seems to be returning all entries regardless of whether it's related or not? What am I missing? Also in case it makes a difference there is one category level but an entry can have multiple categories selected.

{% set category = craft.categories.group('markets').slug('marine').first() %}
{% set products = craft.entries.section('products').realtedTo(category).limit(null) %}
{% for product in products %}
    {{ product.title }}
{% endfor %}
There's a typo in your code :) You misspelled relatedTo. I'm assuming that's the issue because other that than, the code looks like it should work.