夏天 'summertime' ==> 夏天裡, 農事忙, 早榖才收割, 晚稻又下秧.
夏季 'summer' ==> 採蓮是夏季農事，亦是江南舊俗。
Just as "summer crops" sounds more natural than "summertime crops", so does 夏季作物 sound more idiomatic than 夏天作物. "Summer sports" (e.g., Summer Olympics 夏季奥运会) and "summer attire" (夏季服飾) are preferred over "summertime sports" (e.g., Summertime Olympics *夏天奥运会*) and "summertime attire" (夏天服飾). Actually, 夏季服飾 'attire that befits the season of summer' does not mean exactly the same in my mind as 夏天服飾 'attire that befits a summer day or the period of summer'. 夏季服飾 may include summer raincoats, but it would be very strange to me if raincoats are included in 夏天服飾 (適合炎炎夏日的服飾).
Why did I mention 雨天 and 雨季?
1. The reason that 天 'day', not 季 'season', is preferred in xx油紙傘，雨天好伴侣 also applies to xx防晒霜，夏天好伴侣.
2. The 天 in 这里别的都好，就是夏天蚊子太多 'too many mosquitoes in the summertime' is sort of a combination of 季 'period' in 这里别的都好，就是雨季蚊子太多 and 天 'day' in 这里别的都好，就是雨天蚊子太多.
3. The 天 in 好热啊--没办法，现在是夏天嘛 ('It is summertime', 'It is during the period of summer') is definitely not the 天 'day' in *又下雨啊。 - 没办法，现在是雨天嘛*.
請上傳你的夏天照片 'summer day photos' vs. 請上傳你的夏季照片 'photos taken in a particular summer'