Grammar behind “你的脑子进水了”。
At first sight this seems to be a sentence with S-P predicate of type ＂这个士兵胸部受伤了＂or ＂兔子尾巴短＂with 进水 in unusual order P-S (cf. 来了人，发生事故，etc.). A sentence of undoubtedly S-P as predicate type might be 你的脑子水分含量太大 (our brain water content high, i.e. your brain has high water content), a sentence with usual verb predicate might be 脑袋生锈了 (has become rusty，see web), a sentence with nominal predicate (a noun as predicate might be）脑袋一片空白 (mind goes blank). 

On the other hand as stated in the other answer, 进水 is a compound verb, cf. bkrs: 进水 (here) to have water get in (one’s ear, shoes etc), to get flooded;
similarly, 进贼 be burglarized (of a house/etc.), example 我们家进贼了. 

Note however the following examples with 进 being combined rather freely with other objects (and  了 between 进 and object, see jukuu): 


我的鞋里进了沙子. I' ve got some grit / a piece of grit in my shoe.
我眼睛里弄进了一粒灰尘。I've got a speck of dust in my eye.
他的齿缝里嵌进了一小片食物。He had a morsel of food caught between one tooth and another. 


Maybe the last two cases are irrelevant since the verbs now are 弄进，嵌进 i.e.弄,嵌 with complement of result 进. 

Another compound using 进, 进货:  stock (a shop) with goods, 这星期没进货.
Also note (bkrs) 左边进人, 右边出人 apparently confirming the exceptional order P-S). 

There are examples of both 进水了 and 进了水 e.g. 笔记本电脑进了少量水, same for 进贼  e。g。家里进了贼？ 

Note that compounds (复合词）are of the following 5 types:


并列 (coordinate; coordinating; juxtaposed),
偏正 (endocentric),
动补 (verb complement),
动宾 (verb-object),
主谓 （subject-predicate). 


S in the case of 进水 only 4 or 5 could apply; in case of 4 水 would be object of verb 进, case 5 seems doubtful, since in 主谓 compounds the subject seems to always come first.