5. You know swimming against the current is difficult.
你知道的，逆水游泳很难。
还能这样说：
Swimming against the current is rather hard, you know it.
As you know that it is difficult to swim against the current.
应用解析：
make against 有害，不利于;
knock up against 偶然碰见;
over against 在……对面;与……成鲜明的对比
6. Swimming in the sea is great fun.
在海里游泳很好玩。
还能这样说：
It is full of fun to swim in the sea.
Swimming in the ocean is a lot of fun.
应用解析：
fun and games 玩耍;
just for fun 取乐;当笑话;
poke fun at 开玩笑;嘲笑;
in fun 开玩笑，不是认真的
7. Please watch your step.
请注意脚下。
还能这样说：
Please mind the step!
Please pay attention to the ground.
应用解析：
in the night watches 晚上焦虑不眠的时候;
watch and ward 值班，看守;照料;
watch for 等待;当心;
watch out 当心，提防;注意
8. Did you go for a swim in the indoor swimming pool?
你们去游泳馆游泳了?
还能这样说：
Had you gone to the indoor swimming pool for swim?
Could you tell me if you had gone to swim in the indoor swimming pool?
应用解析：
open air swimming pool 露天游泳池;
solar heated swimming pool 太阳能游泳池;
dirty pool 不道德的行为;欺骗的伎俩