When you transplant seedlings into their final outside container, it is good to use light, fluffy soil, but to also do something else.
Give them very little water at first. Do NOT listen to the old wives tale that you are supposed to get the soil really damp then water them in really good right after transplanting. It's much better to only give them a small amount of water then, and continue to only give small amounts at a time for the first few weeks.
Later in the season, it is often better to wait longer and water more each time you do water, but at first the goal to maintain good soil drainage and aeration, is to give the plant roots a chance to grow into the soil around them before large amounts of water start to compact it, so the roots suspend the soil at its present elevation. By doing this you can end up with soil that after a couple months, literally bounces up and down like a sponge because the roots have good structure in it, but conversely, that drains better than more compacted soil, instead of holding so much water like a sponge.
Even then, if you are getting several inches of rain in a short period of time, it is good to move plants to a sheltered area, but otherwise if you are following the minimal watering suggested above, soil that starts out drier doesn't have as much excess when it rains. If pots are just too large to be manageable and the rain just won't stop coming then you can wrap plastic around the base of the plants, over the soil, draped so the rain runs off instead of into the pot. If you have nothing else, garbage bags will work then you can reuse them for garbage when their plant duty is finished. They don't have to perfectly protect the pot from every drop of rain, just to divert a large % of it.